Good luck! I'd like to hear how Arch works out for you. I used to use Arch before I switched to macOS. If I ever get around to using a Pi as a desktop, Arch will definitely my first choice.
GP is not talking about the Arch Linux distro, but rather switching to using a distro compiled for the 64 bit ARMv8 ISA. It’s not a completely accurate comparison, but a fair analogy is armv7h is to aarch64 as i386 is to x86_64.
I've just borked my install trying to migrate to aarch64 from armv7h.
Currently rescuing it (I hope) with a chroot.
Fun! :D
edit: all sorted! aarch64 kernel compiled; migrated the userland over from armv7h (I've been using 32-bit for a few months) - no issues so far! whee!