As part of our recent migration from Sitecore 9.0 to 9.3, we needed to ensure that all user data was up-to-date and accessible. However, some users were created on the old 9.0 site, and their data remained in the old Core database. To migrate only these Delta Users to the new Core database, we explored various options, such as serializing and deserializing users or exporting and importing them using PowerShell. Unfortunately, due to the large volume of users and uncertainty about exporting all user-related data, none of these options worked for us.
We turned to the Sitecore Community for assistance and received valuable support from community members. After considering several options, we decided to write a SQL query to migrate the Delta Users. With insights from Andrew Lansdowne about the tables related to the users and migration process, he shared a SQL script that seamlessly migrated the Delta Users from the old 9.0 Core database to the new 9.3 Core database.
We are grateful for the assistance of Andrew and the Sitecore Community team in resolving this issue. For others who may encounter a similar challenge in the future, I have attached the SQL script used in our successful migration process.