Last I checked, Mercon V costs $4 or $5 a quart, and a pan drain/refill takes about 4 quarts. To change it 4-5 times requires 16-20 quarts, or $64-$80. Too pricey imo.
Here's what I'd do:
1. Drop pan, change filter and gasket, install drain plug. Be sure to clean the magnet in the pan too.
2. I'm reluctant to run the trans pump to get rid of the rest of the fluid. I'd probably mess it up. Therefore, I'd just button it up and refill with 4-5 quarts of Mercon V
3. Smell the old fluid that you just drained out. Does it smell burned? Does it flow like it should, or is it a bit goopy? If it doesn't smell burned and flows like it should, I wouldn't worry. Just run the vehicle as normal and enjoy being done.
4. If and only if the old fluid smelled burned/behaved abnormally, then I'd run the vehicle a few hundred miles and then smell/inspect what comes up on a dipstick check. Does it still smell burned/look wrong?
If not, leave well enough alone.
If so, drain, refill, run a few hundred miles, and smell again.