My guess is that some of the posts are in topics that only an admin can write and or read. He won’t be able to access those posts if he is not an admin.
You might be able to do it the other way.
Merge the non admin account into the admin account, then rename the admin account to the non admin account, then remove the admin privileges.
He will still get “Oops” on topics in categories that he no longer has access to.