Craig, not sure if you have figured this out since our last discussion. I've had a similar situation though...
I assign bunches of Opportunities to a manager, who then assigns them to reps. The sender on the attached phone call then needs to be changed, so I created a view that allows me to change in bulk.
I do have to change them individually for each rep, which isn't the most efficient, but it works.
Advanced Find on Activities
Activity Type = Phone Call
Owner = "Rep Name"
Modified By = Current User (or the manager that just reassinged them)
Last Updated = Today
Activity Status = Open; Scheduled
With this search, you will get all the phone calls you just reassigned to a certain rep. You can then change sender in bulk. Thankfully I never have to do this for more than 5 reps. If it is much more, then I probably wouldn't do this.