FR: in-flight weapon substituion

It comes up now and then that a niche weapon wants to have different effects at different range bands. This can sometimes be done with very complicated and fragile substitue or spawn schemes, but it'd probably be much less of a headache if you could make it transform into a different projectile(or set of projectiles) at a given range. This is similar to spawn, but with spawn you have to jump through hoops to ensure the AI still uses it right and keep from having the longer-range stages spawn when your close range shot hits a target.