for D8, How to skip (don't update) some fields from existing entity during import if this field already has a value
To import only a single field, or possibly multiple, use "overwrite_properties". 


  overwrite_properties: If the migration has specified a list of properties to be overwritten, clone the row with an empty set of destination values, and re-add only the specified properties.


destination:
  plugin: 'entity:node'
  overwrite_properties:
    - field_name


Source:  Drupal\migrate\Plugin\migrate\destination