how to make it happen:

All you do is spawn a weapon pad, set the base weapon to something, set the "Configuration" to some higher-req-level variant of that weapon, and then change the base weapon again. The resulting weapon should have the appearance and some of the behavior of the newer base weapon but fire the projectiles of the req variant.

I've tried this and it seems inconsistent; I think it's based on the target weapon. Certain combinations produce a different rate of fire, some produce that and a different zoom-in visual, some produce both of those a different zoom-in visual and sound, and some produce all of those (or some of those) and a change in the projectiles fired. Every combination I've looked at produces a different magazine size and ammunition capacity, and gives the weapon the name of the req variant that was used in making it.

Even so, this is really wild. I really hope this is made more consistent and turned into a feature. This cannot affect matchmaking as far as I know, so at the very least 343 could just leave this bug as it is. (Please, please let us have this, 343.)