but is it possible to copy the fids off 1 drive to the new drive, then attach the other drive and copy the fids from it to the new one?

Yes, as Peter said, you can do this as long as both of the drives were from the same empeg. They can't be from two different empegs, they have to be from the same one. But then that will work.

Note that this information is already in the Drive Upgrade Guide if you read it closely.