If your FAA certificate is valid and current, then you can fly g- reg. under the privileges of that licence.
I know when the differences training rules came in, there were grandfather rights for prior time on that variant. EASA FCL doesn't seem to have any explicit guidance. Possibly annex 3 covers it.
Logically your sign off by an FAA CFI ought to cover you. But logic doesn't always prevail in these matters.