For a large number of placeholder users, users might want to reassign contributions and memberships by using a CSV file instead of the table in UI. With this epic we enable users to download a prefilled CSV template and add GitLab username or GitLab public email on destination instance for each user on the source instance. With that users can reassign in bulk contributions from placeholder users to destination instance users. We also add API support for CSV-led reassignment. <!-- triage-serverless v3 PLEASE DO NOT REMOVE THIS SECTION --> *This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.* <!-- triage-serverless v3 PLEASE DO NOT REMOVE THIS SECTION -->