I didn't switch to synthetic engine/tranny oils until my car had 185,000+ miles. I was also told by another mechanic that leaks may ensue, and at 285,000 miles still no leaks. I've never been told to not change the fluids, no matter the color or miles, just doesn't seem to be common sense.
I think I've read somewhere, "change it hot, change it often." Can't go wrong with that advice.