These questions should really be addressed to Parallels' sales department.
But yes, a purchased license only allows you to upgrade for 1 year, after which you need to pay the SUS (Software Update Service) fee. If you don't pay it, you are not entitled to updates. I think there used to be a way to re-instate a lapsed SUS, which was still cheaper than a whole new licence, but I have no idea if you can still do that.
BUT,
Other than for Unlimited domains, it is no longer possible to purchase a new "owned" Plesk licence. You can only lease them. So unfortunately you can't purchase a new 100 domain licence, or a 30 domain, or a 300 domain. Only Unlimited.
Faris.