Was going to suggest that. I've made 10 work in 11, but it requires messing with cable positions to adjust cable pull, and non-standard indexing. What I've also done, with great success, is remove a cog. With shimano cassettes, just leave one out, really doesn't matter which. My preference is for removing the smallest cog on an 11 speed cassette and using a freehub spacer as an adapter to make the lockring fit the second smallest and still do its job. That way I don't lose my bailout gear or a cruising gear, just one that's only used on descents and maybe in a sprint. 10 speed is narrower, so you MUST ensure that you reset your limit screws on the rear derailer.