Apologies for joining this thread late. I've been after this function, too. However, as with Howard's experience, I couldn't get the plugin to merge records. I ran quite a few different tests for the plugin using Moodle v.3.9.3 on our development server. No records were merged, but it did completely delete one of the users from the system every time, which was not ideal! I'd certainly proceed with caution.
General help
Moving grades from one user profile to another
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