GETTING MY P80 GLOCK 17 FOR SALE TO WORK

I am confused. I purchased a 22lr revolver a pair weeks back, I wished to lengthen the barrel by about 2 inches so I asked the place I could buy a replacement barrel on the manufactures Web page, they said which the gun was registered to me by the atf with a particular barrel duration and when I modify which i would wish a brand new serial quantity