Once registered the device stays registered (subscription or not). Under certain circumstances Support can help you "transferred" a device from your registered account to another registered account (we had to do this with some fgts that were registered under a former employee's email address).
That said, people on the forum indicated that once the device's subscription runs out, a new owner should be able to get the subscription renewed (up to six months) under their own registered account (on the support site). Someone please correct me on this part though.
For what's it's worth, it has been suggested to just create some secondary email accounts (on the support site) and assign them to the devices you plan to "give away".
NSE4/FMG-VM64/FortiAnalyzer-VM/6.0 (FWF30E/FW92D/FGT200D/FGT101E/FGT81E)/ FAP220B/221C