^^ (what aboothman said) - make sure to circulate the heater core as well

You don't need to achieve 100%, but the heater core is probably storing a quart or two. Water wont harm your engine, at least not in the short term. Over the mid-term, it's going to rust out your water pump, etc. Using distilled / deionized water will help reduce these effects, as will various additives. (Which, essentially, is what you're getting by mixing coolant.)

pop the cap and let it idle, and that'll help bubble up any air pockets. check it in a few days to make sure the spare (plastic) reservoir is at the expected line, and you'll be fine.

the old green coolant isn't going to corrupt the new coolant, it's just going to leave behind a few components that deteriorate in undesirable ways faster than the new stuff. if that's 5-10%, your cooling system should be fine. (in terms of life of the engine, it's not enough to lose sleep over.)