When a Summoner uses the Aspect ability to divert evolution points to take on evolutions for himself, can those points then later be diverted back to his Eidolon's pool or are they permanently stuck in his pool? I wish to use Aspect as a way to store points and not have to remove evolutions from my eidolon in order to get a 3 or 4 point evolution.
Yes, you could store the points and give your PC the 1-2 point evolution, and then change them out when you gain a level, just as you can change the delegation of the other evolution pool points.
At 10th level, a summoner can divert up to 2 points from his eidolon’s evolution pool to add evolutions to himself. ... The summoner can change the evolutions he receives from these points any time he can change the eidolon’s evolutions.
(from Eidolon abilities)
... Whenever the summoner gains a level, the number in this pool increases and the summoner can spend these points to change the abilities of the eidolon. These choices are not set. The summoner can change them whenever he gains a level.
Your eidolon's form shifts and transforms. This spell allows you to change any of the eidolon's evolutions by allocating its evolution pool on new evolutions. If you have the aspect or greater aspect ability, this spell also allows you to change the evolution points spent to modify you, including removing or adding points as allowed by those abilities.
The way I've read those is that every time the summoner gains a level in Summoner, he/she effectively resets the evolutions, gains the full pool of evolution points, then spends them as desired. With the Aspect ability, you are diverting points from that pool at any point you could change the eidolon's evolutions.
Thus, when you gain a level and want to store points, you divert the two from the evolution pool. The next time you gain a level, you choose whether you'd like to divert 0, 1, or 2 points with that ability again.
Also with how Transmogrify spell functions, moving points back and forth between Aspect and the Evolution Pool when you level up seems to be completely viable.