I do not see any way that you could do this without costing yourself a WHOLE lot more time and hassle.
You are just going to bite the bullet and move the "live" mailboxes over. Surely the users can do without their email for a few hours.
Jim McBee - MVP, MCT, MCSE Using Exchange since the v4.0 beta in 1995 - Blog http://mostlyexchange.blogspot.com