If there is no leak or blockage in the heater core, I would not mess with it. This is the epitome of "if it ain't broke, don't fix it."

You can simply remove the hoses at the firewall, and flush it with clear water. The other safe choice would be Mikes Citric Acid recommendation.

Unless there are problems of some sort, you can simply flush everything with water and put in fresh antifreeze.

Good luck,