Of course they should be retro-active! That's the whole purpose of it.
Burns, you're right. But maybe some restrictions can be put for those achievements. After all, a difference between mp3s, mp4s and mp5s should be made. Currently the only difference is the xp loss at mp5. This needs to be changed. And maybe start with those achievements.
Let me give an example to explain better:you're eligible for 3 WPs but you're mp3. You should NOT be able to get those 3. You can get 1 or maybe 3. You move to mp4, you get the 3rd one. And so on. Of course mp5s should not be restricted at all.
Also, I hope that Wish Shop will get new features soon. Otherwise we'll have too many WPs and nothing to buy....ok..nothing too useful to buy