The replacement for Mako is most likely Greg Baldwin. He has been a student of Mako's voice since 1977, and therefore is a great replacement. He has been working with avatar for some time. It is interesting to note that the first time Greg Baldwin speaks in Avatar is in the middle of Iroh's story in 'Tales of Ba Sing Se', right after the window was smashed in the earthbending soccer game. If you pay attention, you can tell that his voice is different from Mako's. The first time Greg Baldwin was credited as Iroh was in the credits of the short 'School Time Shipping'.