Converting from stand alone to domain se

Situation: system setup as 3 stand alone servers, acct, main and backup. I need to convert the system over so that the other features will be available such as login hours ect. This requires a Re-install of NT but it has warned me that i will lose all users and permission info. is there a workaround using the SAM from the ERD? or other means? I have already converted the backup to domain server but it won’t allow being on the same network name and i can’t afford to take the whole system down for a long period of time to make the changeover.