long, long trip with old truck

I have always heard if you change the tranny fluid you need to do it regularly if you haven't done it then don't. This comes from guys who work their trucks daily. I guess the reason being once you open the system it is never the same. Sounds pretty strange to me, but that's what many have said. I change mine every fall no matter the miles, but my truck always has a work trailer on it or pushing snow in the winter.