An SSD is a drive as far as the OS is concerned so it should work.
I know Apple wouldn't do the swap under warranty, because the SSDs aren't certified by Apple (and warranty replaement is like-for-like). No idea whether the reseller would do it: If so, there would probably be a charge as it's not a "warranty" replacement which again I'd suspect would be like-for-like.
I have just phoned the Service Centre (as know the local Apple Store) won't install anything not Apple.
They asked for the seral number but said could not install the SSD as this i7 iMac was missing a part to fit the SSD in. I expect they mean the bracket.
My relative is likely to go for a Crucial SSD. Are all brackets the same that come with SSD