You should have the packages in your cache, if not the Artix package archive would have them. From the look of it you won't need to deal with the lib32-udev conflict as you don't have that. Here's how I downgraded then upgraded again, just tested the procedure and it all went fine here:
$ cd /var/cache/pacman/pkg
$ ls |ag udev
$ sudo pacman -U eudev-249.7-2-x86_64.pkg.tar.zst libeudev-249.7-2-x86_64.pkg.tar.zst lib32-eudev-249.4-2-x86_64.pkg.tar.zst
:: eudev and udev are in conflict. Remove udev? [y/N] y
:: libeudev and libudev are in conflict. Remove libudev? [y/N] y
...
error: failed to commit transaction (conflicting files)
lib32-eudev: /usr/lib32/libudev.so exists in filesystem (owned by lib32-udev)
lib32-eudev: /usr/lib32/libudev.so.1 exists in filesystem (owned by lib32-udev)
lib32-eudev: /usr/lib32/pkgconfig/libudev.pc exists in filesystem (owned by lib32-udev)
(failed, got to remove lib32-udev, you might not have this package installed, note '-Rdd' is a command to use with caution!)
$ sudo pacman -Rdd lib32-udev
$ sudo pacman -U eudev-249.7-2-x86_64.pkg.tar.zst libeudev-249.7-2-x86_64.pkg.tar.zst lib32-eudev-249.4-2-x86_64.pkg.tar.zst
:: eudev and udev are in conflict. Remove udev? [y/N] y
:: libeudev and libudev are in conflict. Remove libudev? [y/N] y
(worked now, also hooks get run including mkinitcpio rebuilds - perhaps this part could be causing your problem somehow besides any changes to udev?)
Rebooted, still working fine.
$ sudo pacman -Syu
:: Replace eudev with system/udev? [Y/n] y
:: Replace lib32-eudev with lib32/lib32-udev? [Y/n] y
:: Replace libeudev with system/libudev? [Y/n] y
Packages (6) eudev-249.7-2 [removal] lib32-eudev-249.4-2 [removal] lib32-udev-250-1 libeudev-249.7-2 [removal] libudev-250-2 udev-250-2
Re - upgraded, rebooted and it still works.