I am afraid you got caught by the Xara Update Service. If you are out of cover then you only get your original download, even if there were a number of bug fixes available.
I believe the latest version you could have had would be v12.8.1. If you designed in that then v12.6.0 will moan about your designs being from a later version.

I believe there was a free upgrade path to v15 so it might be possible to download that version and use the same email and licence number to get that working. Do not uninstall the one you have; both will work alongside the other.
https://www.xara.com/us/designer-pro/download/ does not show v15 so ask for a download link.
Personally, I would raise a ticket with Xara/Magix and explain the PC had to be reformatted and why should you be so penalised.

Acorn