MojaveRed the next major version is not scheduled for release for at least 8-10 months.
Also, when people purchase close to a new release (paid upgrade), they will get larger discount for upgrades based on the proximity to the release. For extremely close proximity (say a few weeks), the upgrade will be free or you only pay a price difference if there is. (Further details will be published when we actually get close to the next major release.)
Bottom line: we won't make you pay double, just because a new version comes quickly after a purchase. 🙂
For any other situation, contact us and we'll be happy to figure out a solution that works for you.