Revision: If it's to fix stylistic flaws or whatever, fine. Some people might object, but only people that you could dismiss with a wave of the hand and an airy "Pshaw!" But if it's to put in plot points that you didn't think of back then, CHEATER CHEATER CHEATER.

Long-running series reaching a conclusion: The only thing that comes to mind is Brust's Khaavren novels; I still half-expect another one to come out some day...