My niece is currently studying undergraduate at a German university majoring in physics. Can she switch to medicine? Does she need to take FSP to switch to medicine? Her high school diploma is not accepted in Germany. She had to take the FSP before getting admission to that particular university.
First of all, the relevant difference is whether she has the "allgemeine Hochschulreife" or the "fachgebundene Hochschulreife (Physik)" as a consequence of the examination she went through. The former means that she is qualified to study any subject at University, the latter is specific to physics (and thus wouldn't allow her to study medicine). The paperwork she got when she did the FSP should say what is the case.
Second, admission to study medicine is highly restricted in Germany. To be competitive for the standard route, her highschool diploma would not need to be considered as equivalent to Abitur, but to an excellent Abitur.
Her university might have a few spots reserved for people on non-traditional education paths or even specifically for people wishing to change subject. Prior to the Bologna reforms, for example, there were a few spots to study Medicine in Freiburg for people who had completed an excellent Vordiplom in a science there. But again, this would not be a straight-forward choice.