What make/model/year is it?
In general, I think FLUSH procedures for old transmissions are bad ideas, as they free all of the worn metal/gunk at once, which can be very bad for modern auto trannies. Also, there is a good chance that some of that gunk is keeping your old seals from leaking.
Drain/fill procedures are terrific for A/T, as the old fluid is very likely dirty and no longer performs within the spec it was made for. It is even a good idea to drain/fill it more than once (with a brief drive in between) to get as much old fluid out of the torque converter as possible, though you will never really get all of it.
I ask about the make/model, because 99-04 Honda Odyssey's have notoriously-weak transmissions and should be treated with kid gloves. DO NOT ever tow with one! If you are going to the mountains, make sure its shifting well. Personally, I would install an aux transmission cooler on any minivan, but especially that generation of Odyssey. If it slips or hesitates going into second, you might have one of the bad, recalled tranny. Do a little research on your VIN with Honda.
Good luck!