Welcome to IRV2.
I've found that generally if the antifreeze needs changeing, it's also time to flush the system. My technique is to look up the factory specs for how much coolant the system holds. Then, drain everything - radiator, block drain plugs, rear heater plugs, etc. Then I insert a tee and backflush everything with good old plain water. Keep the cap off and run it to open the thermostat. After everything comes out nice and clean, shut it down and let it cool. Then drain everything that you can out. You'll never get it all out but what's left is goodclean water. Then I add 1/2 of ther cooling system's capacity in straight antifreeze. After that I top off with water and the end result is a 50-50 mix. I've found that no matter how much you drain a cooling system, there's always something left. If you add a 50-50 mix it'll get diluted once it mixes with the old stuff (or water) that's hiding in the hard to get places of the cooling system. By flushing with water, the only thing left to hide is clean water and by topping off with pire antifreeze you'll have a good 50-50 mix that isn't diluted. It is critical to know your system's capacity though.