They offer monthly subscriptions AFAIK, so I'm skeptical they'd upgrade for just any subscription. Otherwise, a person could buy one month, they have to upgrade the car, and then they never buy a second month.
But many people tell tales of "Well, we won't upgrade your hardware unless you actually pay for FSD first", which is NOT what was sold. At that point you have a case against Tesla for a material difference. Having the hardware necessary for FSD is not in any way contingent on you having purchased or subscribed to the software.