Only thing I see out of that interview regarding anything changing is player appearance upon transformation and that it'll differ slightly based on player race. There is nothing in there that says the demon changes appearance as your meter builds. The devs comments about multiple different demon models were really clear that they had to settle on one design and that they can't have multiple demons out at once due to system limitations.

Primarily because of Alliance Raids and places like Eureka/Bozja having a lot of players and the game can't handle that many models.

Plus it's very clear the demon is limited to oGCD and burst mode appearances unlike the pet/minion natures of SMN/MCH/DRK.

All Yoshi has really given clarity on is that it very much is going to be a builder spender job. Which will be different from Dragoon's gauge maintaining aspect.