I decided I didn't want to export all the mail from the cloud mailbox using Outlook, remove the Office 365 license (or just the EOL license) from the user, then use Powershell to permanently delete the mailbox, then migrate the on-premise mailbox to the cloud, and then re-import the exported data to the new cloud mailbox. I knew that would work but seemed to roundabout. What I ended up doing might have been more so, but here's another way: