Deployment: Changing a Field Type Between Master-Detail and Lookup Is Not Supported for Test Deployments
If you change a field type from Master-Detail to Lookup or vice versa, the change isn’t supported when running a test deployment (validation) to avoid data loss or corruption. If a change that isn’t supported for test deployments is included in a deployment package, the test deployment fails and issues an error. If your deployment package changes a field type from Master-Detail to Lookup or vice versa, you can still validate the changes before deploying to production. Perform a full deployment to another test sandbox. A full deployment includes a validation of the changes as part of the deployment process.