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Do you have any kind of basis for how much it will benefit the search engine rankings? If you were moving from "parts.com" to "carparts.com" it is likely you would see a noticeable improvement for exact match searches. Moving from parts66.com to rvparts66.com presents a complication. Even an "rv parts" search would not be an exact match as you have the "66" included in the domain name. Your site will never have an Exact Match Domain which receives the highest benefit. Instead you would be seeking to move from a partial match domain to a better partial match domain, and the improvement may not be noticeable. I would suggest substantial testing and research prior to making any change. I am not aware of any case studies on a change similar to the one you are considering.