I mistakenly assumed that the Coyote engine I got for my swap had no coolant in it and I used Motorcraft Gold instead of Orange. This weekend I got around to flushing the coolant out with straight water and drove for a few days. I'm working on getting the water out now, so I drained the rad/degas bottle and got probably 6-7 quarts out, disconnected the t-stat housing and got another 1-2 quarts. I filled the system back up and have been driving it for a few days, tonight I plan on draining the rad and degas again. I don't think you will need to be as thorough as I have been, I'm just being a little overcautious because I don't want mixed coolant in my system.
I think if you do the drain, fill, drive, drain again, refill you will have replaced the majority of the coolant.