The next time you buy and set up a new Mac make sure to migrate data and apps from your previous Mac to it right away during the initial setup. It can be tempting to see what it’s like to use it fresh from the factory or to delay migrating because doing so would force a macOS upgrade but waiting is a mistake.
The problem occurs when you do real work in an account on the new Mac, when it comes time to use Migration Assistant to bring over data from your old Mac there’s no way to merge the old and new accounts.
The best workaround is to make sure all important data on the new Mac is also stored in a cloud service like iCloud Drive or Dropbox. Then replace the new account on the new Mac with the old account from your previous Mac. You can then bring all the data back down from the cloud afterward.