So what happened here is that your repository was previously on one of our smaller plans which don't have 'User Roles'. This means that everyone was considered an Admin. So once you repository was upgraded to medium, anyone who was an Admin was moved to the newly available Writer role.
This is by design, to give clear feedback on the fact that 'User Roles' have been activated by showing 'Publish Now' as 'disabled' and that after the upgrade they'll need to contact the repo owner to edit the user roles.
Our product designer doesn't have a strong opinion either way to allow either Admin or Writer after upgrade, but moving 'non-owner' users to 'writers' we can allow everyone to realise the roles have changed as described above.
Let me know if you have any further questions about this.