One more item to add to the bottom of your upgrade checklist test your Administrator login password after migrating and converting.
I recently upgraded a client from 4.30 to 4.50. Installed fine, passwords fine on both versions. Client only wanted to move some of the companies to the new version so we migrated the list they wanted to upgrade. Oops, missed one. Went back to migrate the missing company and my Administrator password no longer worked in version 4.50 or 4.30. Somehow the program wiped out both passwords. Sage tech support said they had heard of this before but didn't know the cause and we had to go thru the whole reset password procedure.
Now I wonder how many of my passwords are gone and I just don't know it yet since I didn't test on my previous upgrades.