seriously, do it at least 3 months ahead of schedule. if you think about it this makes sense. prep courses usually go 3-4 months unless they're accelerated or special and they encourage people to sign up at the beginning of the course. even if people aren't doing prep courses, mcat prep usually takes 3-4 months.
I tried to switch 2 months out and ended up having to FLY TO A DIFFERENT STATE TO TAKE MY MCAT. I DO NOT RECOMMEND THIS. the day before the mcat they say you should do absolutely nothing / do whatever you need to do to be relaxed. I was sobbing in an airport security line because I was running straight from lab, terrified I was going to miss my flight and my MCAT. I wouldn't wish this experience on anyone, not even the worst gunner in life.
Good luck. the actual MCAT is stressful enough - plan for it accordingly so that you minimize the pain associated with it. Even though I still did well, I wince whenever I think back to that day.