TL;DR: Arch-OpenRC is deprecated; all work is now focused on Artix Linux
Because seeing is believing
Proof of concept
I set up an Archlinux guest in a Virtualbox VM with Mate and LXDM. Since this was just for demonstration purposes, I didn't even bother to have a user other than root nor did I configure it in any way other than enabling dhcpcd. I assembled a shell script with the appropriate commands from Migration and ran it. The only extra package was displaymanager-openrc, to have the display manager launch after boot. The VM's hostname was "systemd"; after the operation it became "openrc".
The actual procedure took 35 seconds, including the time to refresh the pacman database and download the openrc packages. The entire procedure (boot to systemd - migrate to openrc - reboot to openrc) took 1 minute and 20 seconds. That's with booting twice into Mate desktop.
I repeat, this is NOT a complete setup. Follow the Installation and Configuration steps or don't come back ranting about how the internets died on you.