Other metamagic feats don't have to specify because with a wizard they are only able to use them on prepared spells (which can only be known spells) and sorcerers can only use them on known spells at the time of casting. This feat gives the wizard the ability to spontaneously transform any prepared spell into any other. I assumed that it would have to be a known spell but I always watch out for rule-bending loopholes...not because I use them but because I've had enough players who have tried to.
What I mean about the spirit of the wizard is that a wizard was designed with spells that required significant time (like 15 minutes) to cast. The spell has a interrupt point at which the wizard can "hold" the remainder of the casting until later (when the spell is actually cast).
This feat allows a wizard to not only expel the magic of a spell without casting it, but allows him to convert it into a new spell which he can prepare and cast in 6 seconds which is about 150 times faster than it would normally take.
If this is the concept of magic (which is the default D&D definition) then this feat allows them to do something impossible: Accelerating the casting of a spell to superhuman levels. Other metamagic feats don't change the preperation (read: prinicple casting) of the spell but rather allow the amount of the spell cast during the prep to be changed:
Still spell allows the wizard to precast any remaining gestures without triggering the spell.
Silent spell allows the wizard to precast any remaining words without triggering the spell.
Quicken spell allows the wizard to leave only an instant of the casting left over but it is very difficult.
The only feat that adjust the preparation of a spell is Spell Master which only negates the need for the spell book to prepare a spell. It must still be precast to be effective.
IF however, your concept of magic is very different from this one, then the feat might make sense. I still think that it is potentially an overly potent feat for a 2 level difference. Unless the DM is very controlling, a wizard will gain access to a fairly large list of spells, any of which could be accessed at any time.
In addition, this feat description leaves no indication of what happens with spells that already take 1 full round or more to cast. Do they take only 1 round, do they take more, or can they not be used with this feat? If the first, then what is lost other than the higher level slot if a Monster Summoning spell is used? If the second, then how much longer? If the third, then specify.
If the feat is internally consistant with your campaign's style of wizards, and if you feel that 2 levels is sufficient, and you clarify the vague points, then I think it could be boarderline balanced. I am still not a fan of the feat and feel it is risking being overpowered.
Hope this was enlightening.