tn mac,

You are right, the new fluid mixes with the old. Drain and fill is a recommended procedure. Both drain and fill and flush are shown in the service manual. There is no recommendation or cautions on mixing the trans fluid or concerns on not draining the torque converter.

I looked at the fluid on the dipstick prior to changing. It was still red and clear. I spoke to the manager of a transmission shop that I know and trust. His shop doesn't do flushes. He said on a new trans, he would be okay with doing a flush, but it isn't necessary. Just don't go any longer than the recommended maintenance period.

As for comparing the trans drain and fill with changing oil without changing the filter, you don't have the internal combustion process going on in your tranny. I got your idea, just not the same.