I'm new to the forums but a long time Taylor owner so I can probably shed some light. The grand auditoriums are fine to use with medium gauge strings but as has already been said, are set up from the factory with lights. Typically when switching to mediums, the nut slots need to be enlarged a tiny bit and the truss rod tightened to compensate for the increased tension from the strings. The top itself is a lot stronger than people give credit to and I've spoken with JR @ Taylor about this. He suggested that it's not a problem switching to medium gauge.
If you want to back to lights, try it first and see how it plays. You'll definitely need to tweak the truss rod a bit and maybe shim the saddle slightly. The nut
should be ok but it may be best to take it to a luthier and have them do a complete set up. And yes, a set up typically involves truss rod adjustments, nut and saddle adjustments... everything to get your Taylor to play like butter
