Note: Product must be same version edition to transfer the license if the service plan is expired on the old serial license.
ONLY if the old company had the latest edition would you be able to transfer the license to the new company by Clicking Help--Upgrade your Sage 50 product, for step by step instruction see related resource article 10334
DocLink: How to upgrade from a lower edition of Sage 50 to a higher edition or change the number of licenses?