Draining the complete system just to swap expansion tanks is a bad idea if you don't have to. Once you've add that much air to the system you could spend quite a bit of time and energy purging enough air just to get flow into the radiators(!), and even then you'd have bubbles to purge that could vapor-lock it a few hours or days after the initial purge. (That's why automatic air scoops are handy- even if there's a bit of gurgle on the system it'll usually burp the rest eventually as long as there is flow.)
Air in the system is hard on pumps, and the fresh water has oxygen in it to corrode stuff, whereas the "dead" system water has very little. Save as much of the system water as you can, and purge air only as-necessary.