Are you still using K2.net 2003 or is it K2 blackpearl.  In blackpearl I would go to the K2 Workspace and under worklists redirect their tasks to their replacements.  I would then turn on their OOF and delegate their tasks to a replacement for any new tasks that might popup because of old assignment settings.

K2.net 2003 had an OOF component but I can't remember exact where.  In K2.net 2003 it would automatically redirect tasks.