What car computer change can cause to the car?

now i'm going to buy car which has changed computer in the front of the car where motor is (i mean bort computer) and has changed another computer in the back of the car where the battery is. so now im interested about problems which that "computer change" can cause in future. the car is 2003 e cl W211 CDI. i dont know exact car terminology, sorry for that, but i think u'll understand what im talking about. if u dont tell and ill explain it further. thx