Si el usuario tiene una suscripción activa el botón de cambiar suscripción, el cual envía al carrito la orden de cambio de suscripción en lugar de agregar una nueva suscripción.

woocommerce-subscription.php

		add_filter('woocommerce_before_single_product_summary', 'prefix_subscriptions_checkout_button', 10, 2);
//woocommerce_add_to_cart_validation

function prefix_subscriptions_checkout_button( $args = array() ) {

    $button_arr = wp_parse_args( $args, array(
        'variation_id'  => 0,
        'btn_class'     => array( 'button', 'button-primary' ),
        'btn_text'      => __( 'Test' ),
        'btn_atts'      => array(),
    ) );

   // $button_arr['btn_url'] = do_shortcode( '[add_to_cart_url id="' . intval( $button_arr['variation_id'] ) . '"]' );

    if( is_user_logged_in() && function_exists( 'wcs_get_users_subscriptions' ) ) {

        // Grab an array of user subscriptions
        $user_subscriptions = wcs_get_users_subscriptions();

        if( ! empty( $user_subscriptions ) ) {

            // Array( 'Subscription ID' => WC_Subscriptions Object );
            foreach( $user_subscriptions as $user_subscription_id => $subscription ) {

                // Loop through the users subscription order items to get the subscription order line item
                foreach( $subscription->get_items() as $item_line_number => $item_arr ) {

                    if( $user_subscription_id == $item_arr['order_id'] ) {
						
						$prod_url = add_query_arg( array(
                                            'switch-subscription'   => $user_subscription_id,
                                            'item'                  => $item_line_number,
                                            '_wcsnonce'             => wp_create_nonce( 'wcs_switch_request' )
                                        ),
                                        get_permalink( $product_id )
                                    );
						?>
							
						<script type="text/javascript">
					window.onload = function (){
					var testlink = "<?php echo $prod_url?>";
                    document.querySelectorAll("form.cart")[0].action = testlink;
					document.querySelectorAll("button.single_add_to_cart_button.button.alt")[0].innerText = "Cambiar de Suscripcion";

					}
					
						 //productUrl +"?switch-subscription="+ switchSubscription +"&item="+ item + "&_wcsnonce=" + wcsnOnce;
		</script> 
						
						<?php
                       

                    }

                }

            }

        }

    }

}