It's possible to change your conversion's external id in the right-hand side details panel that appears when you click a conversion. Once here, simply click on the original external id. This opens the possibility of editing this field. 

Remember to hit your Enter key to save these changes.

It's possible to change your Customer's  customer id in the right-hand side details panel that appears when you click a Customer. Once here, simply click on the original customer id. This opens the possibility of editing this field.

Remember to hit your Enter key to save these changes.