You can update content in Craft once it's been migrated. You just need to rerun a migration to update migrated content.
Click on the '# Items' link
Select which Items you want to migrate, and click 'Migrate Items'
You can search by Item name, and workflow status.
This will update the pages in Craft, which have been previously migrated. Once the migration has completed, you'll be shown a summary of all the Items which were updated, and be able to visit the updated Item in both Craft and GatherContent.