That previous site mentioned also has pages for all kinds of Märklin digital conversions, including this nice page on the 60903 and 60923 upgrade kits.
These two conversion sets are identical when it comes to the five-star high power motor itself. The big difference is the digital decoder.
In the 60903 it was (not produced anymore) a so called c91 decoder, with a row of eight switches to set the decoder address and two potentiometers to adjust the maximum locomotive speed and the acceleration and deceleration time.
In the 60923 we find a mfx decoder. It means, among others, that the decoder is adjusted from outside, from the train controller. You can set a lot of things, like the address (if you use a Delta controller or the 6021 Control Unit), the maximum speed, which can be set rather accurate, the acceleration and deceleration time can be set individually and you have some control of the different functions of the decoder.