1. For a deployment with a new Master-Detail field, soft delete (send to the Recycle Bin) all detail records before proceeding to deploy the Master-Detail field, or the deployment fails. During the deployment, detail records are permanently deleted from the Recycle Bin and cannot be recovered.
2. For a deployment that converts a Lookup field relationship to a Master-Detail relationship, detail records must reference a master record or be soft-deleted (sent to the Recycle Bin) for the deployment to succeed. However, a successful deployment permanently deletes any detail records in the Recycle Bin.