Smart-detect or manually input delimiter when changing Custom Field type into the Labels type.

I imported my CSV data as a Dropdown field, but it turned out a Label field would've been perfect. So I need to change the custom field type, but it cannot use a delimiter to help split the data in one option. For example, the original Dropdown option is "1,2,3" and the new label is "1,2,3", while I want it to be three separate values: "1", "2", and "3".