Yeah, it's never been implied in canon, we just have some behind the scenes quote from Roddenberry regarding TMP.
So anyway, for those of us who follow the old definition of canon, as canon stands now, the ship goes through three refits.
This ship in 2254:
Becomes this by 2256:
Then becomes this by 2266:
Then finally becomes this in 2270:
I guess it's possible. I just wish they had kept the nacelle pylons straight, then it would have been pretty much perfectly acceptable with a bit of head justification. As it stands now, they replaced the entire warp engines twice, with the second refit going back to straight pylons.