Good morning,
I did change the net-vm with a Debian 9 template that I created for that. I followed the instruction here:
https://www.qubes-os.org/doc/anonymizing-your-mac-address/
https://www.qubes-os.org/doc/template/debian/upgrade-8-to-9/
I created a template only for the net-vm since I already had a Debian 9 template with a lot of package not needed for net-vm
Hope you succeed!
Dominique