I have a CMMG 9mm conversion kit & was wondering if there's anything I should be aware of if I wanted to move the front sight forward & then get a full length FF handguard to build a dissy style upper....

If you want a standard front sight base you would have to send it out to have the barrel drilled for the taper pins.  Easier way would be to get a front sight that clamps onto the barrel or one that attaches to the forward end of the FF rail.

Seydou

Nope, nothing really to consider.  I did it myself with my RRA, but only to see if I liked the Dissipator look in general.

I used a Yankee Hill folding front sight/gas block, but any clamping front sight or gas block will do.  Armalite makes a clamp-on A2 FSB, but they're next to impossible to get hold of as Armalite always has them on backorder.  DSGArms may have them in stock though.

Since it's a 9mm, and as such you're not using the FSB for anything other than a FSB, I would maybe even suggest the evil "Blackthorne" website that sells cheap clamp-on front sight bases for $25.

The only other item you'll need is a triangular handguard cap.

good luck