If you're asking about an in-place upgrade you could upgrade directly. In-place upgrades are never recommended due to risks and possibility of corruption carry forward. If you must then I'd make sure that server is patched fully and health is 100%
https://docs.microsoft.com/en-us/windows-server/get-started-19/install-upgrade-migrate-19
If you're migrating active directory domain services then first I'd check that DFL is at 2003 level (or higher) then
use dcdiag / repadmin tools to verify health correcting all errors found before starting. Then I'd stand up the new Server 2012 R2, 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 verify health, when all is good you can decommission / demote old one.
Regards, Dave Patrick ....
Microsoft Certified Professional
Microsoft MVP [Windows Server] Datacenter Management
Disclaimer: This posting is provided "AS IS" with no warranties or guarantees, and confers no rights.
