Woohoo

Would you say changelings are overpowered? 

Sounds like one of those Logic Paradoxes, like "If God can do anything, could he create a door he could not open?"

Overpowered? Maybe... Though I don't expect a changeling taking the form of an alicorn to have magic that's as powerful as Twilight's. But if one decided to practice it while in the form of an alicorn, they might reach that level. Think of how they impersonate others, for example – we've seen changelings that just repeat others' sayings verbatim ("They're changelings, remember") while others (Chrysalis, Ocellus and several others) can do it more convincingly. Maybe it's the same with all changeling skills...