I have a 277-volt fixture that GE put a multi-volt ballast. will I have to change the cap in line with the socket to make it work properly, or can I leave it how it is? Merc reg.

Can’t really understand what you are trying to do. Are you looking at the idea of replacing the 277v ballast with a multi volt ballast?

So my understanding is that you want to run a 277v multi volt fixture on 120v correct?

That's really easy on modern GEs, all you need to do is to change the Faston connector on the ballast primary from the "277v" position to the "120v" position. On modern OEM GE gear, it's typically a blue plastic Faston connector. No need to replace the capacitor in line with the socket.

What Joseph_125 said is correct...you shouldn't have any problems.