Let me get to the point on a couple things
1. Torque converter isn't a closed system. Fluid runs though it from the oil pump in the trans. It absolutely has to, otherwise the thing would overheat and boil the fluid in very short order. Torque converter is the primary source of heat in an automatic trans
2. The whole bit about flushes vs. Changes is tiresome. Sure there are some power machines that hook up to the cooler lines, but even still, those are uncommon in my experience on account of the labor involved to use them. And even still, they pump fluid at low pressures, below 60 psi, which your transmission does anyway.
3. There's this odd double think bit on the forum where the preferred method of changing fluid is to remove the cooler line and start the car (which I disagree with) while simultaneously saying that doing the same thing with a machine which adds fluid at the same time do avoid running thr pan dry is bad
4. You don't need to get all the old fluid out. The system is not that sterile
5. If you ever find yourself at a point where you don't want to flush/change/drain/whatever nomenclature you want because of fear that sludge will be moved around, then I suggest you do some reading. Transmissions don't undergo any combustion and as such, do not form sludge. The only debris that can form in a transmission are wear metals, which will be attracted to magnets at the bottom of the pan, and clutch material, which will be suspended in the fluid. If you change your fluid and find that your trans is ******* up after, it's no fault of the fluid change/method, you'd just discovered that your trans was EOL and was set to blow up within the next couple thousand miles anyway. If your reason for changing fluid is because of poor shifting/slipping/etc, there's a good chance fluid won't help it on account of damage to the hard parts already being done
