I would sit down and start by pulling out a spread sheet and start cataloguing your requirements. I would also take the time to do a full inventory of what you are running now. Then you can map this against the various products available, of which some I list below. Don't forget a skill analysis and map the gap and what that will take to ramp up on the new stuff as well or bring in a partner to help.
if your users are small O365 is a pretty big bargain. If you are using Exchange as part of the SBS deployment, you have to factor that into any move away from SBS. O364 gives you that for about $20 per user a month for up to 50 users, and it also includes licenses for MS Office use.
As for your AD, you can Join SBS to another domain or add a new server to the existing one however you can run 2012 functional level because all your servers of course wont be running server 2012. Also note that if you move domains, there is a tremendous amount of work to do to keep things working in SBS and it must hold all the FSMO roles.
If you want to take advantage of the new offerings I would look to migrate of SBS all together as @Toni has stated. Windows Server Essentials 2012 is for up to 50 users, but there is no Exchange component, MS expects you to signup for O365 for that. You can install Foundation on the product and run it, and there are scenarios where you may want both Essentials server and O365, and MS makes integration between them fairly painless.