If you want to replace the fluid in the converter, you can drain the pan, refill with a minimum amount of new ATF, run in neutral for a few minutes and then drain again. That will get the remaining fluid in the converter and the rest of the tranny diluted with flesh fluid to the point of insignificance. Unless you are dealing with contaminated fluid. Then, you can pull a line off the radiator and flush and flush that way until it comes out clean before you pull the pan.

Mixing different versions of the proper fluid is okay. Frankly, in a stockish application, there is no advantage to synthetic. Any ATF you buy today has a large amount of man-made materials (synthetic) in it. In a stockish application, synthetic vs petroleum based fluid is just a matter of percentages.