Incorrect.
Strike 1 for jackson.
Yes, you can switch projectiles with Kornman's Guerilla, located here:
http://h2v.halomaps.org/index.cfm?pg=3&fid=2230
Ive switched many projectiles around, let me tell you a couple of things.
1. Do not release the map/tag having the projectile swapped weapon, as it is considered noobish by the modding community.
2. Certain projectiles cannot be switched, and some can. (The phantom turret projectile cannot be swapped, however a rocket projectile could replace a smg projectile. I have no idea why, but it has to do with the shared and single_shared maps.
PM me for a tutorial on how to do this.