What to do if a contact has resigned and joined another company

What to do if a contact has resigned and joined another company

Let's say that a contact has resigned from his current company and joined a new one? Let's say he actually had activities linked to him and his company.
Should I edit his contact and join him to the new account or should I create a new contact?
If I edit the contact, what happens with the activities connected to him, his former potentials and his former company? Will they stay with the former company or migrate to the new one (which should't happen)?
Please advise!