We are currently using XenServer 6. Under what conditions can we upgrade to XenServer 7 free of charge?

You can upgrade to XenServer 7 (licensed by socket) free of charge from a XenServer 6 edition (Advanced, Enterprise or Platinum, all licensed by host), if the existing XenServer 6 licenses are under active Citrix Subscription Advantage. Each per-host XenServer 6 license is converted into two  per-socket  XenServer 7 licenses. The trade can take place directly in the “Citrix My Account” (citrix.com) page. There you should click on “All Licensing Tools” and then “Upgrade”.

Source: Licensing FAQ Citrix XenServer 7.0 as is 2016-08-24