According to the posts on SCS forums at the time and now here, the text read
"Upgrade your current version of VEGAS Pro to VEGAS Pro 13 now at an unbeatable price and receive VEGAS Pro 14 following release at the end of September 2016!"
"Upgrade" is also used in the current MAGIX VMSP/VMSS offer at least in the "Net Notify" notice.
Maybe it is just a case of MAGIX using "upgrade" the way SCS used to use in the first sentence below, if they do not ask for a previous version S/N.
It was just that SCS had a special "upgrade pricing" available as per the second sentence.
"Upgrades are new numbered versions which include major enhancements and new features. This upgrade pricing is available to you as an owner of previous or limited feature versions of the application."