Workstations in remote site don't know about new AD realm

I'm going to shoot in the dark here.

Your Samba servers can be WINS servers. I'd consider turning on WINS server functionality on a test Samba DC, passing out its address as an NBNS server (option 44), and telling the clients to do H-node type NetBIOS name resolution (option 46, value 8). You'll have to make a static WINS entry on the Samba server if it doesn't resolve the old NetBIOS name already.

I'm not sure what exactly you've done. If you've stood up a separate parallel domain, you're in for some work. If you've added samba4 DCs to your samba3 environment and then just decomissioned the samba3 DCs, then you're in good shape.

Assuming you've done the second option, it's as simple as pointing the clients to DNS servers that host the zone(s) for your directory. You're smart, so I assume this isn't the case or you would have figured this one out by now. :)

Assuming you've stood up a separate environment in parallel, you'll need to do a migration. In the Windows world, there is a tool for this. It will transparently translate file permissions, user profiles, etc and then change the domain membership. Will this work with samba DCs? I have no freaking clue. If this isn't a viable option, then you may be stuck unjoining and rejoining, or scripting something with netdom