Aug 24 2018 10:38 AMSolution
Hello @Robert Lien,
Essentially what matters here is the license.
If you just convert the user over to a shared mailbox, but leave them licensed, the onedrive data will stay in place.
In your case, since you want it removed, just remove the license from the user as well once it is converted. It will then start the autoremoval process that would happen as normal, which typically is 30 days, and grants access to their "manager" in active directory to get any data that you need.
You can read the full details of the removal process here - https://docs.microsoft.com/en-us/onedrive/retention-and-deletion
So in short, just unlicense them and it will take care of itself.