I dualbooted debian and arch, but I ran into some issues:

Point EFI back to primary boot loader

Debian registered its grub in the efi boot loader list, so I had to change it back to arch's grub.

Adjust to new disk UUIDs by fixing fstab

Once arch booted, it was stuch at the fsck screen for a while

Regen grub.cfg

Debian obviously wont be in the old boot loader, so its config with os-probing needs to be rerun.