After restoring the old templates in Qubes OS 4.2 from backup, you can manually change the repositories in the templates from the Qubes OS 4.1 ones to the Qubes OS 4.2 ones or run this script inside templates to change them:
qubes-dist-upgrade/scripts/upgrade-template-standalone.sh at main · QubesOS/qubes-dist-upgrade · GitHub
If something goes wrong and the templates won’t work then you can still install fresh Qubes OS 4.1, restore the qubes there and try to upgrade to Qubes OS 4.2 again.
Many thanks! I will try that.