The only reason to change coolant is to put the additives back into the system. additives such as anti-corrosion, water pump lubricant, etc. The antifreezing property would not change for many, many years.

 

I asked my mechanic to change my coolant to 100% water + water wetter while he was installing my cams. This was at 34k miles. It made a huge difference. The car takes way longer to warm up in the mornings, plus I don't hear the radiator fans working "hard" anymore. The car doesn't go past 190*F while idling.

I won't neglect my cooling system anymore. From now on, 10k coolant changes for me.