Questions
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What is the risk of changing underscores to hyphens in URLs that are ranking
You pretty much nailed it. If the only reason is cosmetic (which I doubt has any influence on anything), there is absolutely no reason to risk losing top 5 rankings. Good luck! Side note.. Google's official stance is that 301s pass 100% authority but not worth testing. You are guaranteed to see a dip in rankings/organic traffic after you set up the 301s.
Technical SEO Issues | | OlegKorneitchouk0