RE: Transfer Fortiguard?
Everything Dave said is spot-on. I want to add, you might be asked for proof of purchase ( PO#, Invoice,etc.....)
Another option, is for the master to create a sub-acct w/limited access for you and that one device. When the subscriptions expires just have them request thru TAC to transfer it to your own acct.
We did something like that when we sold assets off to another company when we split. This was much simpler than trying to sort thru 80+ devices.