Update migrations to import new content
You could use the track_changes option, ex.

source:
   plugin: d7_node
   node_type: article
   # Allows to import changed as well as new records from the source.
   track_changes: true


https://www.drupal.org/docs/8/api/migrate-api/migrate-source-plugins/overview-of-migrate-source-plugins#track_changes