I asked our dealer about a ATF change, they said it would be $1,000 and would void our warranty . . .
Time to find another dealer. That's Bee-S. Sounds to me like they want a tranny job just outside warranty.
Honestly, it depends on what you're after. If you want the box to last as long as the engine, at least a couple of changes along the way is usually helpful. On our old 855, we changed the ATF in the AW every 30k. At 250+k, it still shifted like brand new.
Otherwise, following the OEM recommendations should usually get you to 100k, unless you are really abusing the thing or it's a really bad design (some old FWD Ford boxes come to mind). If you flip your cars every 5 years, or before the powertrain warranty runs, then just do that.
The VAG DSGs call for 40K changes. They then upped the warranty on DSGs to 100k. VAG is now thinking about moving the fluid change to 50k, and some users have gone over 100k on the original juice, and their boxes seemed OK, at least at that time. Half the people doing DIY changes on DSGs perform no recalibration, without problems.
Most ATXes generate most of their contaminants during the first 5-7k. If you change it out then, you can usually go a good long time thereafter before the next change.
Unless it is presently acting up, you shouldn't hurt the tranny with a fluid change, especially not at that mileage. I'd be shocked if that tranny can't be re-calibrated.