How can the TTL of current records in Google Cloud DNS be changed? The TTL of the zone can be changed through the gcloud dns records --zone=zone edit command, by changing the ttl without adding any records. For existing records, do we need to delete then re-add them?
To edit an existing record, you are correct that you delete the existing record and add the updated version. See the example in the documentation here: https://cloud.google.com/dns/records/#creating_editing_and_deleting_records
If you want to edit another record, put it in the 'deletions' list. Also put it in the 'additions' list and update the TTL there.