Not much info to go on but... ok as this is a work machine I'll assume you made an image before the upgrade so (
i would);
- insert a new hdd... formate it gpt then boot the back up 8.1 image onto it.
- open disk manager, make some blank {un-allocated} space and then install a fresh WX onto that space.
this will give you both systems (
activated) on the same machine in a standard dual senario.
the main point here is that your wx upgrade worked so you DO have a wx key and Microsoft is fine with you running 8.1 as well as 10... I have 8 and 10 on my laptop without issue and 8.1 isn't that much difference to 8 in the boot up.
p.s. whats the real important program that doesn't run on WX... not facebook or the porn network is it?