Query for old posts and update their date fields. Requires the wp-cli-restful package (https://wp-cli.org/restful/).

Republish WordPress Old Posts using WP-CLI

# Linux
wp post update $(wp rest post list --before="`date -u +%Y-%m-%dT%H:%M:%SZ -d "1 year ago"`" \
--format=ids --per_page=1) --post_date="`date '+%Y-%m-%d %H:%M:%S'`" --post_date_gmt="`date -u '+%Y-%m-%d %H:%M:%S'`" \
--post_modified="`date '+%Y-%m-%d %H:%M:%S'`" --post_modified_gmt="`date -u '+%Y-%m-%d %H:%M:%S'`"

# Mac OSX
wp post update $(wp rest post list --before="`date -v -1y +%Y-%m-%dT%H:%M:%SZ`" \
--format=ids --per_page=1) --post_date="`date '+%Y-%m-%d %H:%M:%S'`" --post_date_gmt="`date -u '+%Y-%m-%d %H:%M:%S'`" \
--post_modified="`date '+%Y-%m-%d %H:%M:%S'`" --post_modified_gmt="`date -u '+%Y-%m-%d %H:%M:%S'`"