I would remove the barrel from the stock and soak the end with the nipple in a small can filled with some sort of penetating oil. WD-40, Kroll, even motor oil. Let it sit a while. Then when you're ready, put the barrel in a vice and with a good tight fitting nipple wrench try and remove it. Heating it might indeed help.

Also depending on the make of the rifle.. all nipples are not the same so be sure you have the right nipple (new one) for the rifle.