I went from the full version of Vegas Pro 9, upgraded that to VP10 and then skipped two reiterations and upgraded from 10 to VP13. What I'm not sure about is how the full version license to upgrade license works as far as Sony is concerned. I only thought of this now because a friend had an issue with another companies software and running both the initial program version 10 and the upgrade license version 11 at the same time (he would get a license in use error), though I've never tried that with Vegas.
How deep is the upgrade license attached to the last full version I purchased (which would be VP9)? Just in case I ever lose my VP9 key and access to the Sony account for some reason. Just thinking about trying to surrender my license back to Sony and sell VP13 one day makes my head hurt because it's an upgrade from an upgrade. If there's a VP14 in the works, I'd like to purchase the full version of that, I'd feel more secure that it wasn't tied to an outdated version of the same software. Maybe I'm wrong though, maybe the license functions as a full version license after the initial purchase from Sony? I dunno.
Your new version is a full complete version independent of the version you used to get the upgrade discount.
Your old license remains valid and you now have an additional license.
If the serial number has been used for an upgrade, then it cannot be used for an upgrade again, but you can continue to use the older version(s).
If you sell a license of a version that you do not need, you must contact Sony's customer service and inform them of the transaction. They'll disable your serial number and issue a new one to the other person. He/she cannot get an upgrade discount if you had upgraded from that version.