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Yes there are differences and what do you mean convert it? Are you asking how do you stick a S4 (series 4, 86-88) motor into a S5 (89-92)? or visa versa?
search the 2nd gen tech area. Many people have done this successfully.
you just have to know which parts to swap with the motor and which to use from the other version. There are some wires to splice and clips to connect...