if you upgrade version 6 to version 7 then you need your version 6 serial number to get the upgrade discount and the new serial number for version 7 at the lower price - but you do not need version 6 installed on your computer, you just install version 7 and use the new serial number you bought at the discount price

X3D6 is yours forever... but you may have difficulty getting it to work on windows 10 [don't know about windows 8]

afaik X3D7 is also yours forever, but I don't have it to read the EULA [licence]