I don't know about the 1200 defender, but I have the 1300 defender and if I were to replace the barrel with a longer barrel, I would have to change the magazine tube to a standard tube as well because of where the underbarrel band is located on the defender. (my defender has a longer mag tube than a standard hunting model, which is where a longer barrel would come from).
You might want to check this for yourself.
I myself, will be getting a longer barrel and mag, when I get a chance, for my 1300.