In the thirteen years I've been using this software, the generous licensing model hasn't really changed at all, so no, nothing is moving. You don't have to upgrade if you don't want to, and your purchased version will not expire.
The discounted prices, and their expiry dates were communicated via email, and, as you say, yearly, not twice a year. Though that time span is not set in stone, it's around about yearly. Subscriptions expire when you stop paying. The version of media center you buy does not expire. Upgrades are not compulsory.
These are facts, not fan mail.