Failure Analysis (Delete Assignment - asset group from Failure set)

HI

We want to change assignment of asset group to another Failure set

e.g. motor assigned to failure set 100 and i I want to change to 200

The system does not allow me to do it

this is a message that i receiving after  trying to delete Assignment - asset group from exist Failure set (100) Further to assign to 200

"Failure set association referred in one or more work orders."

all of the work orders with  asset group that assigned to 100 in status "closed" or "complete no charge"

i need a help What can I do to Delete Assignment - asset group from Failure set