It sounds like it might be the datasource for the admin database that is old. If that's true you're nearly hosed, but not quite.

Try doing a search of the properties files in the WebSphere install directories for the old username and/or password. If I remember right, I think that the admin userid and password were actually stored in plaintext, and thus you can change them in the properties file.
Kyle
------------------
Kyle Brown,
Author of
Enterprise Java (tm) Programming with IBM Websphere
See my homepage at
http://members.aol.com/kgb1001001 for other WebSphere information.