I am not sure that it will open the thermostat in the cooler adapter however. Just do a normal oil change, if my memory is correct there is about 4-6oz's of oil in the HD cooler.
Too much worry about nothing. Forget about the oil you can't get to during an oil change. The most important thing is change the oil filter always. That new filter will filter the old oil in a short time and take care of it.