FX impact Barrel swap question

I have an older FX impact in 30 caliber and I just received my 30 caliber X Barrel, now when I swap the barrels out do I have to adjust that knob that's under the Barrel and above the tank? ( the black knob with the lines on it? ) or am I alright?,,,,,BTW I accidentally shot my crony so I have to get a new one before I can even touch that knob anyway. ?