If I buy the X7 version, I can use the serial number to version X6.
Biku.
Bikuina said: If I buy the X7 version, I can use the serial number to version X6. Biku.
No, you will be issued a new serial number, when you purchase the upgrade, that applies only to the X7 version.
But if you are asking if the serial number you have for X6, qualifies you for the upgrade price instead of the full purchase price, the anwser is yes. Do check with the official distributor for your region.