Hi Mike,
In the past, if there have been changes to the licenses, those who have purchased previously keep their existing license structure unless they make changes to their license.
So for example, if someone purchased a yearly license key when there were unlimited sites for a license key, they would keep that option unless they decided to upgrade to a lifetime license as that is a new license key with the new structure of 500 sites.
I hope this makes sense – let me know if you need me to elaborate anything else π