Is this a one-time sale or a subscription?
When you purchase the plugin you get a license that includes 1 year of updates and support. When that year is over the license is automatically renewed unless you cancel the subscription. If you cancel the subscription you can keep using the Premium version you're on when the license expires for free.
After purchasing the plugin you can immediately cancel the subscription by logging into your account. The license key will remain valid until the end of the 1 year period, even if you've cancelled.