User Name Changes

First, a general point: users and groups are not commonly referred to as "metadata" in this niche, that term is typically reserved for dimensionality artefacts (dimensions, members, cubes). So the warning you received is not necessarily related.

On the actual point:

• OneStream stores and references an actual user ID behind the scenes, which won't change when you rename a user or group. As you mentioned, audit records will continue to contain names as they were at the time of the action being audited, but they will also contain the unchangeable ID, so you won't lose traceability.
• the Application Database Filesystem is built around path names, and the user folder takes the name of the user, so anything stored in their personal folder will "disappear" after renaming. In reality, the content will still be in the database, but you might have to manually update tables FileContents, FileInfo, and Folder, to resurface the material.
• The rest of the application should automatically pick up any change, since it's referencing IDs internally.
• If you store things that reference the user in XML format outside of the app (i.e. version control etc), then those things might have to be refreshed after changing username, since they often use names rather than IDs.
• You might want to check any rule, particularly integration/reporting ones, to make sure they don't rely on usernames to look for things.