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.
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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.
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Remember to hit your Enter key to save these changes.