Make sure you have the latest HotSync software (4.1.4e I believe).
Sync the old Treo first to back up all the information for the other agent, then with the correct Treo for that agent just sync and point it to the profile for that agent. It should restore all of the information for him.
On the old Treo that was meant for your boss, you will want to do a hard reset. The information that Palm provides for resetting a Treo requires you to be a contortionist. I've found that holding onto the Power button, hitting reset, then letting go of the Power button when the "Palm" logo appears, works just fine. If you do it correctly, a message should appear asking whether you want to erase all data. Press "Up" on the direction pad to say yes.