Generally speaking, the longer the transmission had been ignored the higher the risk of failure if you change the fluid. At 73k miles it is hard to say how much wear and damage had occurred. If the car was driven very mildly in the past, it could have very little wear. Or it could be worn out already if it was abused. There is no way of knowing for sure since you don't know its history. It's a risk. I would probably give it the benefit of a doubt and change it anyway. If you change it, you might have premature transmission failure. But if you don't change it, you might ruin a perfectly good transmission, which would be more costly in the long run.