You could also do something that will require a little work.
If I remember correctly, when you buy an upgrade license, you enter that number during the installation and it creates a "full" serial number for the new version (in cases such as what you are in), so you'll be able to install v8.6 using that serial number.
If you've ran the bediag.fax on the old server (at any time after backup exec was upgraded to 8.6) you could restore this file (from a different backup exec server, or even NTbackup), the serial number you had should be in this file.