If you hold a permanent license for any previous version of the software, you can use it indefinitely with no subscription necessary.
However, you will not be entitled to new upgrades beyond the valid subscription period you held when purchasing the software (prior to November 2, 2015).
To continue to have upgrades available past your subscription date, you need to update your license and therefore switch to the new subscription-based model.
Under the new subscription-based licensing model JetBrains Account system manages activation and keeps a catalog of your purchases. If your older license version supports JetBrains Account activation (most licenses purchased within the last 12-18 months do), then you will be able to use it. Otherwise, A JetBrains Account signup will be required.