If a single Story has changed scope to be an Epic's amount of work, you can easily convert that Story into an Epic:
- Within a Story, click the gear icon in the upper left-hand corner.
- Click "Convert Story to Epic" from the dropdown menu
- A dialogue will appear: "This Story will be converted to an Epic and then archived. You will be able to find the original Story in a comment on the Epic." Click OK
- Your Epic will be created with a comment regarding the original Story. You can then start adding Stories to your new Epic.