in v2.5 that function was exist and it was very usefful for us , specially when we found a duplicate client account , or if the client want to merge two account they have .
in v3.3 that function not exist , and is planned for future release, and i think is better to talk about it now how it should work .
@PauloV , maybe when mergin clients, the client row should be removed and let the user row or maybe the reverse , in V2.5 the merge clients delete the merged account from the database and add contacts, services, invoices, tickets and transactions from old client to the new client , so no trace for the old user_id .
i think like this scenario is workable .
i vote the paulov idea to quick add this function , because the more plugins/modules created before it , can cause some trouble after it created .
i beleive this function is not present in the core now because for some good reasons, or it just for lock of time ?