Un-assign "Used for variations" from WooCommerce product attribute if the "pa_size" attribute is "Universal size"

woocommerce_unassign_variation_to_attribute.php

<?php
/**
 * Un-assign "Used for variations" from product attribute
 * if the "pa_size" attribute is "Universal size"
 * because of possible multiple version of universal sizes,
 * we search the attribute which slug starts with a "uni"
 */

function unassign_variation_to_attribute() {

	/**
	 * ### Styles for debug output
	 */
	echo '<style>pre{width: 49%;float:left;font-size:12px!important;line-height:1.25rem;background-image: linear-gradient(0deg, #f5d2d9 2.50%, #fafafa 2.50%, #fafafa 50%, #f5d2d9 50%, #f5d2d9 52.50%, #fafafa 52.50%, #fafafa 100%);background-size: 1.25rem 1.25rem;position: relative; z-index:10000; margin:0}</style>';

	// Get all the products.
	$args = array(
		'post_type'        => 'product',
		'post_status'      => 'any',
		'posts_per_page'   => -1, // -1 for all the products, can be performance heavy.
		'suppress_filters' => true,
		'cache_results'    => false, // suppress errors when large number of posts (memory).
	);

	$products_obj = get_posts( $args );
	if ( ! empty( $products_obj ) ) {

		$has_uni_size = false;
		// EACH PRODUCT.
		$_pf = new WC_Product_Factory();
		foreach ( $products_obj as $single_product ) {

			$prod_id    = $single_product->ID;
			$prodname   = $single_product->post_title;
			$product    = $_pf->get_product( $prod_id );
			$attributes = array_filter( $product->get_attributes() );

			// EACH ATTRIBUTE OF PRODUCT.
			foreach ( $attributes as $attribute ) {

				$attr_name = $attribute->get_name();

				// IF ATTRIBUTE "PA_SIZE" and IS SET AS VARIATION.
				if ( 'pa_size' === $attr_name ) {
					// If attribute is used for variation(s).
					$is_variation = $attribute->get_variation();

					if ( $attribute->is_taxonomy() && $is_variation ) {
						$attribute_values   = wc_get_product_terms( $product->get_id(), $attribute->get_name(), array( 'fields' => 'all' ) );

						foreach ( $attribute_values as $attribute_value ) {

							// If attribute term slug starts with "uni".
							if ( 'uni' === substr( $attribute_value->slug, 0, 3 ) ) {
								$has_uni_size = true;
								unset( $attributes );
								continue;
							}

						}

					}
				}
			}

			// Get array of product attributes (PA), saved as '_product_attributes' meta.
			$meta_arr = get_post_meta( $prod_id, '_product_attributes', true );
			// Duplicate array of meta PA.
			$__meta = array();

			foreach ( $meta_arr as $key => $value ) {

				$__meta[ $key ] = $value;

				if ( 'pa_size' === $key ) {
					$old_value = $__meta[ $key ]['is_variation'];
					if ( $has_uni_size ) {
						// ### Output array pair, matching upper criteria - for debug
						$__meta[ $key ]['is_variation'] = '<span style="color:white;background:red;font-weight: bold;padding:0 5px;">IS VARIATION WITH UNISIZE</span>';
						// ### Replace the value
						// $__meta[ $key ]['is_variation'] = 0;
					} else {
						// If criteria doesn't match - keep the old value.
						$__meta[ $key ]['is_variation'] = $old_value;
					}
				}
			}

			/**
			 * ### THE DEBUG OUTPUT:
			 * Two preformatted columns - compare to check
			 * First is the original product meta 
			 * Second is a copy with located values for replacement
			 * when everthing ok, comment debug output and,
			 * uncomment the update_post_meta line
			 */
			// Original meta.
			echo '<pre>';
			print_r( $prod_id . ' -' . $prodname . '<br>' );
			print_r( $meta_arr );
			echo '</pre>';
			// Copied meta, with located values.
			echo '<pre>';
			print_r( $prod_id . ' - ' . $prodname . '<br>' );
			print_r( $__meta );
			echo '</pre>';

			/**
			 * ### CAREFULL: WHEN UNCOMMENTED, THE LINE BELLOW CAN MESS UP
			 * PRETTY BADLY, IF EVERYTHING ABOVE ISN'T SET RIGHT
			 * FIRST, CHECK EVERTYHING WITH DEBUG OUTPUT ABOVE
			 */
			// update_post_meta( $prod_id, '_product_attributes', $__meta );

			// Reset display of duplicated product meta and
			// Universal size existance boolean.
			unset( $__meta );
			$has_uni_size = false;

		} // end of each product.
	}
}

/**
 * We used admin_init hook here,but perhaps would be advisable to
 * simply output the unassign_variation_to_attribute() function
 * in newly created blank template, not used anywhere, especially
 * it this is run in production (live) site.
 */
if ( is_admin() ) {
	add_action( 'admin_init', 'unassign_variation_to_attribute' );
}