Depending on your local water, that might be OK or a real no-no. If the water has a lot of disolved solids, they can degrade the compunds in the antifreeze that protect water passages and radiator from corrosion from the inside.
tonydiv -- any idea how much comes out useing your approach? My HD towing coolant system is supposed to hold 24 quarts dry, according to the shop manual. Obviously, not all of that will come out, but depending on the proportions, if say 75% it is in the radiator, then its not so hard to get at least most of the old stuff out.
I was thinking of buying about 10 gal of distilled water, draining out the rad, refilling will distilled H20, running it till warm, draining again, replace with dist water again, repeat, then one last drain.
Then, based on how much came out, I would approximately know how much water is in the system still. I think I would pour in 3 gal of straight antifreeze, and then top back with more distilled water. If the shop manual is correct at 24 quarts (6 gal), then I should be OK.
I am also wondering about some of the flushes that are sold to clean out the system even better. I did that once on a old Buick and the water pump died 500 miles later. Sometimes its better not to mess with things too much I think.