Password reset for ~1998 AIX on RS/6000?

All, I?m trying to resurrect an AIX workstation that may well contain useful information for the NASA IMAGE spacecraft. The system is an IBM RiscSystem/6000 43P-140 according to the badge on the front (which I think is reasonably accurate). It?s running a version of AIX from ~1997 or 1998. It does boot to the GUI login prompt. Is there a standard procedure for recovering lost passwords for these systems, or for resetting passwords? I do have physical access to the machine; it?s possible I can find an AIX install disk but it?s *highly* desirable to preserve the contents of the existing hard drive. Apologies in advance if this is posted somewhere obvious, I have not done my google homework so a pointer may be all I need. - Mark 210-522-6025 office 210-379-4635 cell