Event's Regular Fees' Fee Labels are re-created (rather than edited) on changes

Currently, when an event has non-Price-Set based Regular Fees and their Fee Labels are edited, the whole set of fees is dropped and a new one is added to civicrm_custom_value. This means that the labels are, effectively, non-localisable - if you edit a label in English and then try to edit it in some other language (say, Polish), instead of civicrm_option_value.label_pl_PL being updated a new row is entered and all label_* fields default to the new, Polish value.