Changes? It depends on your tongue weight. Most drawbars are spec'd for 10" between hitch pin and center of ball. Suppose VEHICLE (not receiver, VEHICLE) is spec'd for 1000# tongue weight. Using a 12" drawbar actually decreases the max tongue weight spec slightly, to 833#, although folks usually ignore that. HOWEVER, using an 18" drawbar decreases the max allowable tongue weight to 555#. As long as that is within your TT specs, fine, but that means a max of around 4273#. How heavy is your TT?
Problem has to do with torque on the hitch to vehicle attachment. Personally I'd remove the tailgate and throw the extension away, but that is just me.