Need to first clarify if you actually have a
Subscription or a
Perpetual license. The latter is part of an
Update Plan, and letting that lapse means you can continue running the versions provided on your perpetual (non-expiring) license. If you really have a
Subscription then that's temporary, so letting that lapse means the license expires and you can't run PT.
With a Subscription, you can
subscribe again any time after it lapses.
With an Update Plan (Perpetual license) you can renew ($99US) while active, or
Upgrade ($299US) later if expired.
Hope this helps.