Trans Service at 203k miles

I know that the transmission fluid change has been discussed before, and there are some good tutorials on the drain and fill and the poor mans flush methods. Here is my question. I bought my 2006 crew cab nismo with 123k. I have no idea if it was ever serviced. Now I'm at 203k. Never having serviced it myself, what is the best course of action. Let her be? Get a flush at a shop? Drain and fill? I have noticed that it is hesitating and possibly slipping when I put the pedal to the floor to pass on the freeway. What should I do?