Do I need to buy both SuperMemo 15 and SuperMemo 15 Upgrade in order to use the newest version of SuperMemo?
No. If you are already a user, you can buy only an upgrade. It is functionally identical and much cheaper. However, if you do not qualify for an upgrade, you will need to purchase the full version. Future updates and patches are free within a single line (e.g. SuperMemo 2008, SuperMemo 15, etc.). In other words, you do not ever need to worry that you will get an earlier version of the same line that might include some undiscovered bugs.