Change note type operates on the note and all its cards - it is not possible to split off a single card into a separate note type. You can export the cards into a text file, and import them back into a separate note type if you wish. If you make a mistake and did not export a backup prior to starting, you may be able to recover from an automatic backup. https://faqs.ankiweb.net/restoring-an-automatic-backup-recovering-from-data-loss.html
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