You should be able to move the service qubes back to fedora 40 with no issues. Probably the appvms too, not the standalones obviously.
If it was me, I’d want the service apps to work as the first step. Otherwise how are you going to get internet access to update or revert the erroneous update that has messed things up
If an update created this issue then I would redo the update or revert to previous one that was working. You could try loading an older kernel in grub to test if that fixes any issues
Look at some of the later cli commands in this thread as a way to fix the dom0 update issues