Keep user account but provision new empty mailbox

Running

Set-User <UserUPN> -PermanentlyClearPreviousMailboxInfo

for a cloud mailbox in EXO shell might kill the entire mailbox.

usually, such mailboxes are exported (e.g. as PST) and parked. other common way of dealing with this is archiving the mailbox (by mailbox archiving solution). but this doesn`t fully separate old from new mailbox data, as long as the user still has the same mailboxGUID, which you might want to recreate afterwards (also be careful with self service archives where user could restore by himself)

keeping an existing cloud mailbox and disconnecting it from the user object that you want to keep (in AD and M365) is only possible, if you fully split your M365 synced user from your local AD user (cloud synced mailbox => this requires you to change proxyaddresses & UPN in cloud after doing so. Also all permissions in SharePoint, Teams are then still bound to the separated cloud only user, as well as all his calendar data and contacts.

IMHO it is always recommended, to create a new ADUser in such cases if your plan is to separate things from another. if you are separating (mailbox) data from old and new user, you usually also want to ensure he has new & clean permissions, proper GPOs and doesn't have access to all files he had in the past

to do so, old User needs different properties (UPN, Mailaddresses), so that they can be attached to a new user.

Also if you create a new mailbox, you might still need to "migrate" contacts and calendar. User could also export those by himself if he was warned and informed how to do so.