for D8, How to skip (don't update) some fields from existing entity during import if this field already has a value How to skip (don't update) some fields from existing entity during import if this field already has a value?
That for Drupal 8 with modules (migrate, migrate_plus and migrate_tools)

example of an yml I would like:


process:
  uid:
    -
      plugin: migration_lookup
      migration: users
      no_stub: true
      source: uid
  field_telephone: phone
  field_user_country: pays
  field_adresse/given_name: prenom
  field_adresse/family_name:
    -
      plugin: skip_on_notempty  \--> do not exist
      method: process
      source: \--> actuel value of 'field_adresse/family_name', How to get that ?
    -
      plugin: get
      source: nom
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