Edit: Should have stated that it's the automatic transmission, which is sealed, but I'm not sure what they consider the useful life when they do that. I doubt it's going to be forever though.

So, I have a 2000 Grand Am GT with ~207k miles on it currently, I bought it at ~164k. I have not done anything to the transmission and i have no information on anything that might have been done in the past.

I have no drivability issues. She shifts now at 207k just like she did at 164k a couple years ago. Currently mostly highway driving at about 300-400 miles/week.

I expect to need to keep this car if at all possible at least until she reaches 250k miles.

Given that, would the chances of a problem before 250k warrant the expense of having my fluid/filter changed? If so, how critical is it that things are perfectly level? Would a concrete floor and eyeballed jackstand height work or does it really call for a properly set up lift at a shop?