Migration from Windows NT to Windows 2003 server enviroment

Hi, I have joined a new company in which the environment is Windows server 2003. They had migrated from Windows NT to 2003. The confusion I have is, their previous domain name (before the upgrade was JSRM01)but after the upgrade to Windows 2003 they have changed the domain name to XXX.com but still when the users log in they log in to the JSRM01 domain. When I checked the DNS it has all ip lists in XXX.com. So can you please tell is this an usual behavior? If yes/no please explain ASAP!!!!!!