I guess that's the magic of the Arch Wiki¹. Even if not using Arch I often find it far more useful and up-to-date than the wiki of my own distributions (Ubuntu and Manjaro).

[1]: https://wiki.archlinux.org/index.php/Migrating_between_archi...

On a multiarch distro, like Exherbo or Debian, you can switch architectures by cross compiling for the target arch.

On Exherbo, after cross compiling, one only needs to change the /usr/host symlink and reboot.

I've done that a couple of times over the years, on Debian systems. Largely as a result of moving from UML->KVM.

This is roughly the guide I followed, which has existed for a good few years.

https://wiki.debian.org/Migrate32To64Bit