J, just my opinion, but if the pine treads are solid an 1 1/8th isn't much to lose compared to the effort needed to dig out the old treads. Kinda doubt you'll notice much difference. Otherwise you'll almost need to gut the walls to get to the old treads enough to tear them out and nail in the new ones. I'm betting the side walls are built on top of the treads and you may have a devil of a time even getting the old ones off. Have you considered going with something less thick? If applying over the solid old treads you really don't need a full thickness new tread. Good luck.