I always love it when IT decides on moving files around servers. It always makes more work for those that don't have the time already on producing plans. I prefer mapped drives for this issue.
The problem is the drawings are flagged as being included in an existing sheet set.
Anyway, you could modify the old Sheet Set file to point the old files to the new file location. Unfortunately, this is a time intensive process. I believe JTB World's SSMPropEditor program may help in this situation.