Hi CP62 -
I agree with Huge: the Nova should just drop in as a replacement for your SuperUniti with no need to adjust your existing storage and streaming practice. It will see your NAS and UPnP server and operate in the same way with the same (next generation) control app.
If you want to switch to local storage with an attached USB drive, I would suggest that you use a software conversion solution such as dBpoweramp or XLD or EAC to make a WAV copy of your existing FLAC library and put that version on the local disc. This is easy to do, and can be done in batch mode (specifying the source and target paths for your existing FLAC and new WAV libraries) Then you could compare and contrast the sound quality and convenience of your streamed and transcoded FLAC files against the locally sourced WAV files.
That said, it is certainly not a requirement for listening on Nova or any of the other Uniti products. Indeed, you can try doing this now with your SU and check it out (use a FAT32 drive, treating it like a large USB key) and see what you find. Then you'll have both solutions in place when you make the jump from Super to Nova.
Regards alan