Gnome-boxes, how to switching from rpm to flatpak and retain vms

I am on Fedora32 and used the preinstalled gnome-boxes so far and have created a couple of vms that I also want to work with in future.
Now, I found out for installing Windows 10 enterprise in a boxes-VM I needed a newer version of gnome-boxes (F32 rpm has version 3.36.6) I installed gnome-boxes from flathub (version 3.38.1).
I did this on another computer (also running F32) first and managed to setup the Win10VM there, all fine.
Now, I wanted to do this also on my computer with the existing VMs. However, after installing the flathub-gnome-boxes my existing VMs are not displayed any more.

I found out about the different pathes:

  • rpm-boxes has the images in ~/.local/share/gnome-boxes/images, and other stuff in ~/.config/gnome-boxes and `~/.config/libvirt
  • flatpak-boxes has everything in the respectively named folders below ~/.var/app/org.gnome.boxes/
    However, when I place a copy of my original files into the respective folders und ~/.var, gnome-boxes starts and displays “Oops, something went wrong. Boxes cannot access the virtualization backend”.
    How can I make my old VMs available in the new gnome-boxes flathub-version? Is this even possible or do I have to newly setup all the VMs?