My thoughts were that lets say you are running CB9.5 now and buy the CB10 upgrade but do not use it (do not activate the code) then when another new upgrade/version is released, lets say CB12, then as long as you activate your CB10 upgrade within the CB12 grace period or after then you will have a CB12 system.
This works well for bi annual upgrades as you can purchase the cheaper .5 upgrade rather than the full point upgrade. Of course you will always be tempted to just activate it so that you have the latest software each year 