I'm using the old conventional Prestone green coolant (ethylene glycol) in my 21- and 12-year old CRV and Kia Sportage. I know I'm supposed to replace the coolant every 3 years at most. The danger of this stuff is silicate dropout if left too long in the car. My question is: if I drain the radiator only and do not do a flush to get the coolant in the engine block out, and then refill only the radiator, will the old and new coolant work together and give a year or two of adequate protection? Or will the old coolant behave independently of the new coolant and still precipitate the silicate?