Trouble with draining from the sump plug would be that once the oil starts flowing out it will be very hard (and messy) to replace it while the oil is flowing out without oil going everywhere. It may be easier to drain completely into a clean container, replace sump plug and then refill from the top. Someone told me that some of these quick oil change places don't bother with emptying via the sump plug, but use a vacuum down the dipstick tube (which is why it's bad to use them as they leave loads of crap in the bottom of the sump). They might however be able to vacuum out a bit for you?
