Levy said nothing is changing, just the context. So my understanding is the characters will just default the the alternate adult forms and using the "shrinking potion" or whatever it ends up being just makes them look the way the filly forms do right now, no actual changes, just the context of what's happening. e.g adult scootaloo's mind in filly scootaloo's body vs filly scootaloo's mind in filly scootaloo's body.