They're basically just files so you can move them safely enough, however if you want them to work properly without going through the hassle of rebuild a new VM and attaching it to the old VHDx then you can use Hyper-V to export the individual machine(s) and then when you have Hyper-V back up and running again simply import it or them.
Although again, I've upgraded once already from 8.1 to 10 (10240) and I had no trouble with either Hyper-V or my VMs.
Back on 8.1 again now courtesy of Acronis True Image and I'm gonna do the upgrade to 10 again tomorrow or as soon as it's actually available for me to do so.