There is at least one workflow where a session's type might change. I login (usually tty1) using agetty -> login -> pam_systemd.so. So the session type is Type=tty and XDG_SESSION_TYPE=tty.

I then set XDG_SESSION_TYPE=wayland and start a Wayland compositor (although starting X here would be similar). But I have no way of changing the session type in logind. There are projects in the wild which look at XDG_SESSION_TYPE for wayland vs x11; I'm not sure about org.freedesktop.login1 or sd_session_get_type(3).

Is it reasonable to want a way to change this in logind? Something like loginctl update-session Type=wayland.