There's nothing wrong with having oil left over in your cooler anyways when you do an oil change? It will eventually get completely mixed in and diluted with the new oil... And correct me if I'm wrong, but isn't it said somewhere that there is some benefit to leaving a bit of old oil in anyways to help condition the new oil??
Not to mention your lines are still full and the cooler is full so on the following start you don't have to worry about any loss in pressure due to air in the system or a dry crank.
If you really want to go through the hassle of doing all that extra work, by all means knock yourself out. But If you're going through all that trouble anyways, what's wrong with undoing 2 lines, 4 screws and dumping the cooler the good old fashioned way and refilling it??