The point is that the form of government theorically can already be discussed and even changed, but not changed by amending the current constitution. If the Italian people wanted to change form of government to the monarchy, a new Constitution should be written and approved, and not just the current constitution amended.
Art 139 doesn't say that the form of government can't be changed; it says that the republican form of government can't be changed by a constitutional amendment.
It is pretty much a useless provision, that's true, because no republican parliament would ever dream to amend the Constitution in order to change the form of government. But at the same time it's uncorrect to say that that article forbids discussing and ultimately change the form of government.