Secure access before you announce anything: confirm you hold owner rights on ad accounts, analytics, tag manager, domain, hosting, and business profiles. And export campaign structures, audiences, and creative while relations are calm.
Sequence matters: gain ownership → export → give notice per your contract → transfer or rebuild. If the agency owns the accounts (sadly common), negotiate transfer in writing as part of the exit; ad-platform support can sometimes assist with account moves, but leverage disappears after the goodbye.
Expect a performance dip during transition regardless. New hands, algorithm re-learning. Budget a month of patience into the plan rather than judging the new arrangement on week two.