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Our #1 priority keyword, which we've ranked #1 for years, suddenly we're #2. And, the page listed above us doesn't seem to be even compete. Moz On-Page has us at an A and them at a C.  When I review the html, I don't even seem any exactly keyword match or matching text on the page.  I checked are ranking last week and didn't notice any change - so I've narrowed it down to something changing in the last 4-5 days. Also of note, when I test,  We're #1 on mobile, #2 on desktop. Sorry to not list the url's. omitted intentionally. Any thoughts would be much appreciated.

Any assistance/feedback is greatly appreciated. The scenario: We currently own two website, and we'd like to combine them and eliminate some expenses.  Although the content is very similar in nature, it is not exact. www.KF.com that is managed by a third-party provider & www.KFA.com that is managed by the manufacturer of the product we sell.  (*sites url's are not accurate) We have ended the contract of KF.com, however, this site has the best  SERP/SEO.
We assume we'll take a hit, no matter what we do - however when it comes to SEO, but what is the right move to make? Do a domain "Transfer/Redirect" of KF to KFA.com or Do we simply change the KFA.com to KF.com? Still very much a rookie when it comes to this stuff.  I do have the ability to SEO the KFA.com webiste. Hoping this makes sense - and apologize for the bad url's just not sure I can actually post the true addresses. Thanks in advance.