New customers are on monthly recurring payment, but old customers are on a lifetime of use.
Due to a big success of SS came need for regular updates and support.
The one time fee enables you to use latest ss version that was active before expiration of your support, with your key. And you can continue to install it on your sites and register it without any problems.
Renew option gives you another 365 days of any new versions, updated versions for you to install and, in case you need, developers can help you more if you need.
Other than that, you may continue installing SS PRO on your blogs, without any problems.
Just find somewhere version that you downloaded last, and use your license key to register it.