Well it would stay the same as always. You get what you paid for. Like I can still use R-Mix and VC64 and they still work so that full license payment entitles you to the full plug just like that. If your membership lapses and you go month to month you still own what you previously owned but would NOT get that new updated version.
Don't think anything really needs to change aside from the license/reg code for full and locked versions but if they are going to go through and start locking stuff anyway (which no one from Cake has said but that's what seems to be what is being discussed) then it could be done in this manner.
Full Sonar license = Full unlocked plugs with reg codes for that specific version.
Let your membership lapse = Keep what you own. No unlocked updates until you buy another full license. If you sign up month to month you get the LOCKED version until the full 12 months is paid for.
Really it's just two licenses. Now the code involved in locking things based on reg codes or whatever... that I don't know about. It could indeed be a massive pain in the arse but I'd imagine it's just a matter of auto generated keys and code and whatever.
Cost/Hassle vs. Benefit really which of course would be a large part of the bottom line equation.
Thanks for entertaining the premise anyway. I certainly don't expect my ramblings to affect the future of such things but hey... you never know.
Cheers.