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There were some pre-reqs when I had to do an OEM upgrade from 18.104.22.168 to 13.2, but in your case the OEM version stays the same so it's pretty much a straight-forward/as-per-normal database upgrade.
This series of MOS notes are normally my go-to, so that should be enough
Complete Checklist for Upgrading to Oracle Database 12c Release 2 (12.2) using DBUA (Doc ID 2189854.1)
Hope this helps
Why would you expect there to be any issues?
There's nothing magic about an EM repository. It is nothing more than an application schema, with the application being EM.