I've done this replace a few, add a few, replace a few more, add a few
It took a LOT more time, and required a lot of restricted state outage.
I would have been better off to unload/reload.
If they're scsi make sure that your raid stripe expands over 8 drives. It
will perform better than 4, and the dreaded 2. One other thing to be
careful of when replacing small quantities at a time.