My search for an answer isn't coming up with a hard-set rule, but from personal experience: No, once you've bought the games (even at reduced pricing), they are yours to play and keep.
This is based on a currently lapse Xbox360 account, but also precedent from other services like Steam and PS+. A reduced cost game due to membership is akin to that of a sale price. Even if you only paid a penny, you still paid for a license and that license is the same one as if you paid full price. You will not need to pay the difference.
That said, while your account is expired, you will not get access to any of the "membership only" sale prices as well. (so buying a new game or DLC will be in their full pricing, even if the base game was at reduced pricing for members.)