I got my 3 gallons of coolant from Suncoast and mixed with 3 gallons of distilled water, and did a low temp thermostat at the same time, and pierburg WP. There is a good article on Rennlist on how to easily bled every drop of coolant out but I think it may be 996 specific, but there may be some parts in there applicable to us:
Simple 3-step complete coolant drain (updated w/ refill procedure + water pump tips) - Rennlist Discussion Forums
It is for 996 so the hose routing and stuff may be different enough that it does not apply to us, but I thought it was pretty interesting.
Steve