coolant flush on a4

[no1of consequence]

hey kids,

my new-to-me a4 1.8t has not-red coolant in it. its green. i want to flush 
it out, audi claims they have a pressue flushing machine that gets all the 
old coolant and muck out. but its like 300$ for the service. yikes.

anyone have details on this? is it worth doing? or will the old 'run a few 
engine-fulls of distilled h20 thru it' trick do just as well? is it true 
that the different coolants will crystallize inside the engine if mixed? i 
know bentley says not to mix them, but it doesnt really give a reason.

thanx in advance.
jim