Best way to merge non-profit onmicrosoft.com domain into existing Primary domain

Dave, this sounds more like a tenant migration than a straightforward domain change. In practice, moving between Microsoft tenants can involve users, mailboxes, files, permissions, security settings, and other dependencies. Your existing main tenant already having the users and security structure may simplify some parts, but the migration still needs careful planning. I would also confirm with the migration partner how any TechSoup-related licences or benefits would be handled, as these may have their own requirements. Doing a detailed assessment before moving anything can help reduce the risk of disruption or missing data. Getting a few proposals from experienced migration providers could also give you a clearer idea of the work and cost involved.