Well, there's not a whole lot you can do other than just flush the old stuff out and replace it with new stuff. The little scaly, gooey, ick that accumulates on the walls of the cooling system will eventually flush out. But if you're not leaking or overheating then there's really not that much going on. Conventional green coolant does wear out just sitting in your engine in the garage so, from now on, a periodic change is called for.
Here's a tactic that you (and others) might consider: You can avoid the air bubbles and nuisance of a full drain and refill if you
simultaneously drain and fill three or four gallons of your 50/50 mixture each year. Just fill your overflow tank up to the top, open your petcock at the bottom of the radiator, and drain three gallons out while, at the same time, you pour your fresh mixture in from the top, so no air bubbles get in the system. Just do that once every summer and you'll always be just fine. No, you don't get every last drop out, but the continued dilution of the left over old mixture with the fresh new mixture, will be just as good, if not better than a once every three years cycle.
