We created a cloud only domain for using Microsoft Teams ( managed in azure ad) college.edu but have another domain college.ac which is our primary on-premise domain managed in AD. We did this because we were not ready to migrate our mailboxes to the cloud or use our AD to manage cloud users at that point. So each user has an edu and an ac upn one for the cloud and the other on-premise
Now we have decided to migrate our on premise accounts to the cloud using an express migration procedure and then manage all users in Azure AD. Once we do this, we would like to merge the two accounts for each individual retaining the "ac" (previously on-prem) domain as the upn.
We could discard the edu address but that will mean recreating over a 1000 groups and mailboxes on cloud would be lost. (losing the mailbox on cloud is a compromise that is acceptable, if there is no way of merging the two upns in the cloud. Please note we do not want maintain a long-term hybrid co-existence and this merge needs to be done only after we have disconnected the on-premise infrastructure.
We have adequate licenses for short-term co-existence of the two domains.
