Don't worry. Tage is our best forward either way and ever since he was shifted to C he's shown chemistry with pretty much everyone he's played with.
He'll get his minutes no matter what line he's on, in fact, his line will likely be considered the top line, regardless.
By the same token, I'm pretty sure, Cozens won't lose his efficiency just because he plays the wing.
In this hypothetical scenario, it's about mixing and matching the wealth of young high potential forwards we already have in order to bring out the best in everyone, after all, Granato's best feature as a coach so far has been doing just that.
So why not let him try to do with Casey what he did with Tage.
Also league-wide, it's not uncommon to have one of your best forwards on a line other than the "1st line": Malkin, Pasternak, Draisaitl, Tkachuk, Necas, P.Kane, J. Hughes,... to only name a few.
Finally, a coach isn't married to his line combos for the whole season. It's about finding what works and about flexibility.