Funny. This is starting to sound familiar... almost like some other
software that runs at boot, every boot. Hm, what was the name...

Oh, a *bootloader*! Something that *loads* different *boot* configurations!

But seriously. For people that can install a bootloader, is there really
any "reconfiguration" needed to reboot into a different init system?
Just add configuration items as needed for different init systems. We've
never automatically added bootloader options if sys-kernel/* is updated;
why start now?

For those who are on EFI with direct load of Linux, either install a
bootloader or use efibootmgr or similar to add entries into the native
boot selector (really another bootloader).