I have to agree that the wording in the original rules is not a 100% explicit. It is clear that you cannot cast spells of a higher level than you have access to, but can you prepare them? The Pathfinder FAQ has a more complete answer, emphasis mine:

No. You only get the bonus spells if your class level grants you access to those spell levels. You can't even use them for lower-level spells. See page 16, Abilities and Spellcasters section: "In addition to having a high ability score, a spellcaster must be of a high enough class level to be able to cast spells of a given spell level. "

For example, a 1st-level wizard with 18 Intelligence has (according to table 1–3: Ability Modifiers and Bonus Spells) 1 bonus spell at spell levels 1, 2, and 3. However, he can only use the 1st-level bonus spell because as a 1st-level wizard he only has access to 1st-level spells (his class-based number of 2nd- and 3rd-level spells per day are "—", meaning "no access to spells of this level"). As soon as he becomes a 3rd-level wizard, he gains access to his 2nd-level spell slots and can use that bonus 2nd-level spell slot from his high Intelligence, and likewise for 3rd-level spells and bonus spells at wizard level 5.

Basically, ignore the columns for higher-level spells on table 1–3: Ability Modifiers and Bonus Spells until your class grants you access to those spell levels.

The Pathfinder FAQ is considered official in the Paizo community and is binding for Pathfinder Society (http://paizo.com/pathfinderSociety/faq#v5748eaic9qy5).

First, I'd like to say that this kind of oversight and poor editing is common in the Mythic rules, and it's likely that whoever wrote the Mythic Spellcasting rules wasn't thinking about retraining spells when they wrote the feat and power. Thus, any ruling you make on this will likely be supported by at least one interpretation of the rules text.

That said, I would say that the rules as written most support the interpretation of the selected Mythic spells remaining static once chosen. In the description of the feat/power, there is no mention of changing the Mythic spells selected, and generally choices made in feats cannot be changed without specific retraining. This implies that the selection process is something that happens once, when you take the feat, and is static afterwards. Since nothing says that you can rechoose your Mythic spells if you stop being able to cast the regular version, you can't. Those Mythic spell choices become basically inert until you regain the ability to cast those spells, or use some kind of feat retraining to get a different set of Mythic spells.