Using just Mxxx will make you move systems, but you don't get kicked off the planet. I tried it when on Earth once, and it moved me to the new system, but I was still on Earth. I don't know if it causes NCb's to be reevaluated though...
If the set expression is evaluated while you are on a planet, Mxxx and Nxxx don't move you until you leave the planet, as I recall it. Of course, if you absquatulate the player, too, they'd leave as soon as you dismissed the dialog.
Actually, you are moved as soon as the set string is evaluated.
Unfortunately, all I managed to do when attempting to change the available mission with this was to dis able the first mission. The second mission didn't show up until I took off and landed.
