Great question — answering as the UphillTech team:
It is not possible to merge two separate Google My Business listings - Google Reviews are stuck to the listing they were left on forever. You should keep both Google My Business listings active and update the business information on the listing that has the most reviews for the new brand. I would also recommend changing the website URL on the unchanged listing to a webpage on the new website that explains the merger: "Example Company is Now New Company" with links to the pages on the new site that correspond to the services/product pages that a customer looking for the former company would be looking for. You should not create a separate Google My Business listing for the new company...
Full answer by Mike Ciffone: https://webmasters.stackexchange.com/a/136792 (CC BY-SA 4.0).
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