What exactly should I expect when i perform FSBO migration from SBS 2008 to Server 2012 ?

I assume you mean FSMO. Moving FSMO roles is not disruptive.

AD groups do not "live" on any single server. They exist on every DC in a domain. Moving FSMO roles would not change that.

Shutting off SBS will interrupt any functionality still dependent on that server. Which is a much broader question that just FSMO roles. If you migrated everything else, it'd be painless, but you haven't indicated anything in that regard, and your other questions allude to some knowledge gas on a proper migration process. Makes it tough to answer accurately.

Oh sorry ,
I have migrated Exchange
print servers
dhcp
dns
fax servers
file server
setup another DC
Application servers .
and I am looking at WSUS
I am just wondering what may not be obvious at this point

Every network is unique. By "not obvious" it basically means nobody here could guess. Ir...put another way... if we could guess, it would by definition be obvious.

I meant not obvious " to me"   My primary concern is whether this will affect the ability to access the above mentioned services .
I was looking for gotchas that others have experienced.