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.
Acquiring Immunity to Death from Massive Damage Is Weirdly Difficult
Immunity to precision damage or critical hits doesn't make one immune to death from massive damage. Having the elemental, ooze, or plant type doesn't make one immune to death from massive damage. A creature under the effect of the spell gaseous form [trans] (PH 234) isn't immune to death from massive damage. Even becoming incorporeal doesn't render one immune to death from massive damage. Apparently, massive damage is supposed to kill living things, and the way to become immune to massive damage is not to live.
That means immunity to death from massive damage can be gained only from the 7th-level Sor/Wiz spell body of war [trans] (SpC 35), the 7th-level Sor/Wiz spell undead mask [necro] (SS 71), and the 8th-level Sor/Wiz spell veil of undeath [necro] (SpC 229).
The mask and veil leave the target's type unchanged but grant many undead features, including immunity to death from massive damage. The body grants the caster, by turning him into a warforged titan, the construct type, hence immunity to death from massive damage; the caster can't cast--even via magic items--while affected by the spell.
So, um, yeah. That's it. That's all of 'em.
You can go sideways. Use the spell magic jar [necro] (PH 250-1) to move your mind into a body that's immune to death from massive damage or the spell polymorph any object [trans] (PH 263) to change form into a creature (or sandwich) that's immune to death from massive damage, for instance, but of the thousands of Wizards of the Coast spells published for D&D 3.X no spell exists that simply says, "You're immune to death from massive damage."
What You Can Do
- Lug around a talisman of undying fortitude (MIC 188) (8,000 gp; 0 lbs.). As a swift action twice per day for 3 rounds gain immunity to death from massive damage.
- Use the 2nd-level Sor/Wiz spell heroics [trans] (SpC 113) to gain the feat Martial Study (ToB 31-2) and pick the maneuver mind over body (ToB 64). Spend an immediate action to make a Concentration skill check that replaces a Fortitude saving throw.
- Use a wand of conviction [abjur] (SpC 52) (1st-level spell at caster level 18) (or just cast the spell if you're level 18 or higher) and the spells superior resistance [abjur] (SpC 174) and recitation [conj] (SpC 170). In other words, just have a +14 bonus to all saving throws. Then get a golf bag of luck blades with zero wishes (DMG 227) (22,060 gp; 2 lbs.) or a mantle of second chances (MIC 115) (12,000 gp; 0 lbs.) or access to the granted power of the pride domain (SpC 278-9) or whatever to reroll failed saving throws.
Best Answer
It’s impossible to prove a negative, but I’m going to say that I have never seen such an option, and it seems unlikely given Wizards’ design for them to make a generic option along these lines. A feat specific to a certain race or creature, sure: then you can know exactly what it is you’re offering. But if the feat or whatever is generic, then the authors of the feat have no idea just how much DR they’re improving, and they may possibly be unaware how much they are improving it.
For example, if the feat were so broad as to apply to any amount of any type of DR, and add an additional restriction of “-and-slashing” to it, a lich’s DR goes from 15/bludgeoning-and-magic to 15/bludgeoning-and-magic-and-slashing, sharply limiting the number of weapons that can actually bypass it (executioner’s mace from Dungeon vol. 135 is the only wieldable weapon in the entire game that could do it; bite attacks could also). The lich’s DR was always good; this makes it basically perfect.
Potentially worse, though, is someone who had just DR/magic, since usually DR/magic is close to meaningless. Making that DR/magic-and-slashing makes it much more significant. Since DR/magic is close to meaningless, designers are often comfortable putting far larger numbers before than slash than for other forms of DR. Such a feat turns that overly-large number into something very meaningful, and that’s a big deal.
Finally, Wizards was always (overly, really) conservative with DR for players. Since there were relatively few feats written just for the DM, and feats were often seen as a “player thing,” it would be out of character for them to write a particularly flexible DR-changing feat.
Considering all of this, and my rather-extensive knowledge of the options available in 3.5, I’m going to say this doesn’t exist in the general case. There may be options for, say, a shifter or warforged or something specific like that, but not just a general use one. Additionally, lycanthropes never saw very much support, so I tend to doubt there is one just for them. For the sake of double-checking, though, I have looked through the Savage Species feats, as Savage Species is by far the most likely book to find this kind of thing; it does not.