size.php

<?php
function _split_product_size() {

	$term_need_delete = array();

	$q = new WP_Query(array(
		'post_type'  => 'product',
		'posts_per_page'  => -1,
		'order' => 'DESC',
		'meta_query' => array(
			'relation' => 'AND',
			array(
				'value' => 'pa_size',
				'compare' => 'like'
			),
			array(
				'key' => '_solid_checked',
				'compare' => 'NOT EXISTS'
			),
		),
	));


	if ( $q->have_posts() ) : while ( $q->have_posts() ) : $q->the_post();
		$id = get_the_id();
		$old_size = array();

		# Get current terms
		$_temp = wp_get_object_terms($id, 'pa_size', 'all');
		// var_dump($_temp);
		foreach ($_temp as $term) {
			$old_size_id = $term->term_id;
			$term_need_delete[] = $term->term_id;

			$old_size_name = trim( str_replace(' ', '', $term->name) );

			$new_terms = explode(',', $old_size_name);

			$insert_terms = array();

			foreach ($new_terms as $value) {
				$term_id = wp_insert_term($value, 'pa_size');
				if ( is_wp_error( $term_id ) ) {
					$_t = get_term_by('name', $value, 'pa_size');
					$term_id = $_t->term_id;
				}

				$insert_terms[] = $term_id;
			}

			wp_remove_object_terms($id, $old_size_id, 'pa_size');
			wp_set_object_terms( $id, $insert_terms, 'pa_size', true );
			add_post_meta( $id, '_solid_checked', 1, true );

		}

	endwhile; wp_reset_query(); endif;

}

add_action('admin_init', '_split_product_size');