Upgrading allows you to acquire license (and key) for a new major version of the product whose previous version key you possess.  Note that after upgrading you will still be possessing the previous version product key, but it will be changed to the "Not For Resale" status thus making further transfer of this product key impossible.

Upgrades are ordered at the User Area, via the "Upgrade" button placed near the product key for which an upgrade is available.

Upgrades are usually available with 50% discount from the full price.