function pum_theme_migration_1( &$theme ) {

	$delete_meta = array( 'popup_theme_defaults_set' );

	// Used to merge with existing values to ensure data integrity.
	$meta_defaults = pum_get_theme_v2_meta_defaults();

	foreach ( array_keys( $meta_defaults ) as $group ) {
		// Get old data.
		$v1_meta_values = pum_get_theme_v1_meta( $group, $theme->ID );

		// Loop over all fields which were merged and mark their meta keys for deletion.
		foreach ( $v1_meta_values as $old_meta_key => $old_meta_value ) {
			$delete_meta[] = "popup_theme_{$group}_{$old_meta_key}";
		}

		$existing_v2_meta = $theme->get_meta( "popup_theme_{$group}" );

		if ( ! empty( $existing_v2_meta ) ) {
			continue;
		}

		// Merge defaults.
		$values = wp_parse_args( $v1_meta_values, $meta_defaults[ $group ] );

		// Update meta storage.
		$theme->update_meta( "popup_theme_{$group}", $values );
	}

	/**
	 * Clean up automatically.
	 */
	pum_cleanup_post_meta_keys( $theme->ID, $delete_meta );
}