Modifying Field Values for a Custom Content Type?
I just checked a select list I have on a site with text key|value pairs under 'Allowed Values'. Although I had the message at the top that 'There is data for this field in the database. The field settings can no longer be changed.' I went ahead and altered the list, and it took the changes fine. 

There should be no impact on the existing data if you change the order. Just be careful you never delete a value.