Sharepoint 2010 "Person or Group" no longer in Sharepoint Group or AD

We keep a task list in SP2010, and record the team member responsible for the task - current, past, upcoming. Past tasks may include "Assigned To" person who is no longer part of our Sharepoint Group, (changed position / responsibility) or no longer part of AD (left the company entirely). If I edit other fields of a past task - corrections, follow-up, etc. the Assigned To field tries to validate the entry from the old employee and fails. I end up having to "reassign" old tasks to current team members just to save updates. Is there any "override" or "remember past valid selections" setting, or any better practice - let users pick Assigned-To person in 1 person or group field, but store their entry in another field, not validated against either AD or the Sharepoint group?