Unable to collect end date after field is saved
When I did this, I created a new date field and set it as hidden on the page. I configured it the way I wanted, then duplicated the existing information in the new field, manipulating the database directly. As you will have two date values (..._value and ..._value2) you should begin by setting them the same.

Once you are sure that the correct values are in place, you must then change all your views, rules etc that use the date field to use the new field. 

Then set the new field to display and hide the old field. Make sure everything is working before you delete the old field.

There is no short cut that I am aware of. Best to do all this on a copy, with extensive backups. Good luck.