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Changing the field formatting for existing entries
Changing the default field formatting does not affect existing entries.
Troubleshooting
Changing the default formatting options for a custom field at Admin > Custom Weblog Fields will only affect future entries.
If the new formatting is to be applied to existing fields as well, care must be taken to select "Update all existing weblog entries with your new formatting choice?" at the bottom of the page whenever the default field formatting is changed.