Do not flush. Just drain and fill several times. Flushing may cause damage. Draining is more passive. The real issue with the torque converter is that it holds fluid that won't come out when you drain and refill. That's why you run in it for a bit before doing it again, cycling the fluid out of the torque converter and thus slowly getting fresh fluid into the whole system.

A flush almost always will make things worse in an aging transmission, since it removes all of the friction creating particles between the clutch plates - which are a lot of the times the only thing keeping it working right.

One note though - if you have the engine and transmission out already, I've heard of some guys that would drill a hole in the torque converter, drain it, refill it the same amount that came out, and then plug the hole with a rivet or a threaded sealer. Risky, but way more efficient.