I own a European 850i from 1990 with 110k miles (168000 Km). I've just had the 5-speed automatic transmission kind of "refurbished" by an unofficial BMW specialist with no improvement. A BMW dealer ha offered me to replace it for $4000. Since I'll have to shell out big bucks I'm considering the possibility of having it replaced by a 6-speed automatic. Is this possible and/or adviceable, would the electronics or rother have to be replaced at the same time?