Jung believed that psychology was bulit up layer by layer, with things being added though out life. However, nothing was ever removed. Thus, if you wanted to deal with an undesirable impulse, all you could do was add a counter impulse to balance it out. Or to put it another way "You're always an alcoholic, you learn to stop being a drunk".
This explain the idea of fixed type to some degree. As has been mentioned, you learn to use the first two functions first, then then the rest follow - its the same progressive layer build up over time. Changing type would involve removing functions, something Jung wouldn't have believed in.