Found a relevant link. Is the process outlined by @elliot still best practice in ACF Pro? http://support.advancedcustomfields.com/forums/topic/best-practice-for-changing-custom-fields/
To paraphrase @elliot, [write SQL in phpMyAdmin to the tune of…]
UPDATE wp_postmeta
SET meta_key = 'new_field_name'
WHERE meta_key = 'old_field_name';
and
UPDATE wp_postmeta
SET meta_key = '_new_field_name'
WHERE meta_key = '_old_field_name';