I started off using the green Koolance liquid until I let an unused portion sit an sedimentation built up at the bottom. Not a good sign. Since then I've been using their blue liquid in my Danger Den setup an it holds up pretty good. I dilute the liquid with distilled water.
I'd say only change the coolant in the system if your temps go up noticeably or if has been 1 year since your last change.
It'd probably be worth redoing the tubing if there is any noticeable build-up of gunk, and clean the blocks too.