As much of a believer in changing tranny fluid as I am, I too would be hesitant to change it, especially if it has never been done before. My guess is changing it at this point would probably do more harm than good by loosening varnish and deposits that have accumulated over time.
I would vote to leave it alone and be happy it still works great.
My wifes 00 C230 is a sealed unit as well and I've changed it every 30k miles. Watch hers fail before it gets as many miles on it as yours. That would be a dose of irony.....