No, adprep is now a part of domain controller promotion.
The prerequisite before introducing the first 2016 domain controller: domain functional level needs to be 2003 or higher
I'd use dcdiag / repadmin tools to verify health
correcting all errors found before starting
any operations. 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.
Also at some point either before or after (if not already) I'd recommend migrating sysvol replication from older FRS technology to DFSR
--please don't forget to
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