1. Create a CSV file to update users;
2. Go to "Upload Users" page, select file, click "Upload users" button;
3. Change the "Upload type" from "Add new only ..." to "Update existing users only";
4. Fields "Allow renames" and "Allow deletes" remain disabled

Problem exists when using IE7, ok in Firefox, not tested in other browsers.

Tested in various moodle installs. Tested with various IE security settings.