From the Player's Handbook, pg33:
A good cleric (or a neutral cleric who worships a good deity) can turn or destroy undead creatures. An evil cleric (or a neutral cleric who worships an evil deity) instead rebukes or commands such creatures, forcing them to cower in awe of his power. If your character is a neutral cleric of a neutral deity, you must choose whether his turning ability functions as that of a good cleric or an evil cleric. Once you make this choice, it cannot be reversed. This decision also determines whether the cleric cast spontaneous cure or inflict spells (see above).
It says evil clerics rebuke undead and spontaneously cast inflict spells. While it does say that neutral clerics who pick one or the other can't change their mind, it does not say that your abilities don't change when you change alignments.
To borrow a computer science term, it's stateless: it doesn't care about what your alignment was in the past, only what your alignment is now. If the cleric is evil right now, he rebukes undead and spontaneously casts inflict spells. If he doesn't like that, he needs to move back to neutral and then choose turning undead & spontaneous cure casting (note that this choice does have state: the rules says it cannot be reversed, so any time he moves from good/evil to neutral, he uses whatever he picked the first time he was neutral).
Generally, template describes the situation if there will be any Hit Dice size change . Wizards online Types and Templates defines this as follows (along with changeing Constitution score)
Before applying any Constitution changes, check the Hit Dice and Hit Points entry in the template. Some templates change the number or size of the creature's Hit Dice or both. Some templates change previously acquired Hit Dice and continue to change any additional Hit Dice the creature gains. Most templates, however, change only the creature's racial Hit Dice (that is, the Hit Dice it has before adding any class levels). Most templates are fairly explicit about what happens to the creature's Hit Dice, so just follow the instructions in the template.
When you know the size and number of the creature's Hit Dice, recalculate the creature's hit points using the modifier from the creature's new Constitution modifier for each Hit Die (whatever its size).
In your example, you re-roll hit dice with new hit dice size.
UPDATE:
From D&D 3.5 Monster Manual Hit Dice entry description:
This line gives the creature’s number and type of Hit Dice (the die rolled to generate hit points), and lists any bonus hit points. A parenthetical note gives the average hit points for a creature of the indicated number of Hit Dice.
D&D generally defines base HD and additional HP (CON bonus and other bonuses if available) for creatures, but also offers a pre-calculated value for HP of that creature. So you can either calculate it or use pre-calculated value.
That means, game mechanics leave the final decision to DM (in most cases), and it is up to him. (Such as; in my games, I generally offer players 3/4 of the die they roll for their HP when they level up, and calculate NPC HDs in a similar way.)
Better you discuss that with your DM and all of your players and find a way that suits everybody.
Best Answer
Note: What the question posits doesn't do what the querent wants it to, but I've gone through the steps anyway so it can be seen where the process falls apart. Skip to the end if uninterested in the process's nuts and bolts.
First, apply inherited templates in a legal order...
Officially, the Dungeons and Dragons 3.5 rules from the Monster Manual for applying templates supersede the Dungeons and Dragons, Third Edition rules from Savage Species, including that text's type pyramid (142-4). An individual DM can, of course, adapt those rules to Dungeons and Dragon 3.5, but needn't (and, I'd advise, shouldn't). The Monster Manual on Acquired and Inherited Templates says that
and on Reading a Template says that
So templates, then, alter the creature in the ways they say they alter the creature. Order can be important, but templates do what templates do.
Anyway, the template half-fey (Fiend Folio 89-90) says that the template
The template unseelie fey (Dragon Compendium Volume 1 222-3) says that the template
The omission of either acquired or inherited is odd as templates are either one or the other (MM 291), so just looking at the template unseelie fey puts the reader in house rules territory. Confer with the DM, but the description indicates the template should probably be an inherited one:
Either way, at least one and I assume both templates must be applied to the creature first, so I'm going to apply these templates to a moon elf, making the moon elf's type fey (augmented fey, augmented humanoid).1,2,3
...Then apply acquired templates in whatever order the DM allows...
Before trying to do more, finish the remaining steps of character creation (PH 6).
Becoming a Divine Minion
The creature gains its level 1 feat during character creation, and it needs to pick this feat:
This (or something like this) must be done because, otherwise, the template divine minion can't be acquired. The Magic Books of Faerûn Web column "Hate of the Cobra (Spells from the Church of Set)" says
And, unfortunately, fey isn't one of the listed types. So the half-fey unseelie fey moon elf from Evermeet cleric 1 of Set after having taken the feat Otherworldly—her type changing to outsider (native, augmented humanoid, augmented fey ×2)—, is then picked by a Mulhorandi god to become a divine minion. That's despite
but whatever. It happened.5,6
Becoming a Ghost
Then the divine minion half-fey unseelie fey moon elf from Evermeet cleric 1 of Set with the feat Otherworldly has adventures until she achieves enough XP to advance to level 2.
The Savage Progressions Web column "Gaining a Template Midcampaign" on the General Rules for Template Classes says that
Now, I don't really know what happens at this point narratively to reflect the game's mechanics because, here, the character's supposed to die then advance as a ghost. That's not usually a choice. A character doesn't just, like, go around stabbing himself and hoping. But let's say the DM approves.
What this also means is that a creature can't enter play with only a template class.7 Silver lining? The template class ghost isn't restricted to specific types like the for-reals template ghost (MM 116-8), instead
Thus, the divine minion half-fey unseelie fey moon elf from Evermeet cleric 1 of Set / ghost 1, upon having gained that first level in the template class ghost, gains the natural ability undeath, and
And, while I know it's silly for, for example, a beholder, displacer beast, or even a divine minion half-fey unseelie fey moon elf from Evermeet cleric 1 of Set to have its type and subtypes changed by the template class ghost first level ability undeath to undead (augmented humanoid), that is what happens, the quotation marks making sure you write that down on your character sheet exactly like it says.
Becoming Dissatisfied
Whatever types and subtypes the creature had struggled to acquire before now are obviated by—I kid you not—those quotation marks in the template class ghost's ability undeath, that specific ability overriding the general rules on Reading a Template (MM 291).
(Note that a generous DM may allow this level 2 creature to use from the Player's Handbook II the rules for Class Level Rebuilding (197-8) to replace that level of cleric with a level of the template class ghost.)
...But this combination of templates doesn't allow the creature enter the prestige class fiend of possession
The prestige class fiend of possession (Fiend Folio 204-7) has as a requirement that the character's race be outsider with the evil subtype. When all's said and done, the divine minion half-fey unseelie fey moon elf from Evermeet or Sidëyuir cleric 1 / ghost 1 is an undead (augmented humanoid).
Alternatives
Notes
1 Keep in mind that some contend the template unseelie fey should have a higher Level Adjustment than +0 (q.v. threads from 2011 and 2013).
2 The creature's challenge rating, however, should be increased by +1 as per the Compendium errata.
3 The DM can look at such a character with a jaundiced eye, however, and totally choose not to allow the half-fey unseelie fey into the campaign. Really, such a DM may opine, can a creature's type change to the creature type it already is? Or, alternatively, can a creature's type become the creature type the creature already has?
4 I guessed that you'd pick elf, but, given the question, who knows?
5 Scholars suspect alcohol was a factor.
6 I'm guessing that the template divine minion was included because of this:
Yet a typical creature has no alignment subtypes to change, and neither does, for example, a half-fey unseelie fey moon elf from Evermeet cleric 1 of Set with the feat Otherworldly. A DM who owes you money might, in exchange for blanking the debt, grant a divine minion a subtype anyway, but as written, the template doesn't grant one.
7 At level 1 a character is created (PH 6); after level 1, levels are advanced (PH 58-9).