When we upgraded our servers from Server 2008 to 2012, I added 2 new branch servers and chose the Foundation Edition because if offered everything we needed at those locations. Now it's time to move to Server 2016 and there's no longer a Foundation Edition! Do I need to upgrade my 2012 servers to Essentials first, and then move to 2016, or is there another way? I'd also appreciate any pointers on 2012-to-2016 in-place upgrades. Buying a whole new set of servers is NOT an option.
You could move the fsmo roles to other server then rebuild them one at a time.
I'd check that the domain functional level is 2003 or greater. I'd use dcdiag / repadmin tools to verify health correcting all errors found before starting. Then stand up the new 2016, patch it fully, license it, join existing domain, add active directory domain services, promote it also making it a GC (recommended), transfer FSMO roles over (optional), transfer pdc emulator role (optional), use dcdiag / repadmin tools to again verify health, when all is good you can decommission / demote old one, and move to next one.