1. Set up a new, temporary, Admin user. Remember the username and password. You're going to need it in a moment.
2. Log out and then log back in again as the new Admin user.
3. Navigate to Users and select your old Admin profile.
4. Reduce your old profile down to Subscriber and save the new settings.
5. Update your old profile back to Admin again and save the new settings.
6. Log out and then log back in again using your old username.
7. Check that you can now see your old Admin name in the Author dropdown in Add/Edit Posts.
8. If all is well, delete the temporary Admin user. You no longer need it.