User Account Name Change Issue

Hello - we have decided that all users in SAGE 500 should use their domain credentials instead of a locally managed SAGE user name. In the past most of our users have been setup with a username in SAGE where the first initial of the last name and the first several characters of the first name become the SAGE login. For example, John Smith would get a SAGE user name of 'SJohn'. At the same time John Smith would have a domain name of 'JSMITH'. For some reason, a couple of our users were given SAGE names that matched their domain name (JSMITH in SAGE and in the domain). When I try to create a new user pointing to the domain user name I get an error due to the duplicate user name.

My question is what steps can we take to address this question for these users? I understand that the current SAGE user name probably cannot be changed or fully deleted. That would seem to leave me with the option of changing the DOMAIN account. However, that isn't my favorite option either. Anyone have a great idea for getting around this?

Thanks in advance for your assistance.