I was kinda surprised to see that some Simulators can be changed quite radically on the fly. A few years i was flying a research session in a
simulator that is normally certified as class D and used for normal flight crew training for the A330 and A340. In that session the simulator was actually simulating an aircraft that doesn't even exist (A340 with a canard above the forward fuselage and A300 undercarriage, the latter one was just cheaper than the original landing gear model, the canard was the research topic) and in the next simulator session a normal crew was doing its normal recurrent training.
To change that simulator all that was needed was plugging in a notebook and change some aspects of the simulation model while still using those modules that were available from the original simulator. On the link given above you can find further information how they do it.