Get a grip!
I've been removing heated grip covers for years. The latest grip type is the most difficult, but heat is the answer. A long ride with the grips on high will frequently soften the glue enough to get the grips sliding around, or careful work with a heat gun. Once the original covers are off, you can install the grip of your choice. The only problem is that you need two throttle-sized grips. I you want to run foam grips, they're inexpensive enough to buy two sets and throw away the smaller grips.