Output a link to switch a subscription for the current user.

switch-subscription-link.php

<?php

// Make sure WooCommerce Subscriptions is active
if ( class_exists( 'WC_Subscriptions_Switcher' ) ) {

    // Get the key for the customer's current subscription
    $subscription_key = WC_Subscriptions_Manager::get_subscription_key( $order_id, $product_id );

    // Check if the current subscription can be switched by the customer
    if ( WC_Subscriptions_Manager::can_subscription_be_changed_to( 'new-subscription', $subscription_key, get_current_user_id() ) ) {

        // Print the link to initiate the switching process
        printf( '<a href="%s">%s</a>', WC_Subscriptions_Switcher::get_switch_link( $subscription_key ), __( 'Choose a new subscription', 'textdomain' ) );

    }

}

// Get plan switch url
  // - https://gist.github.com/thenbrent/7454868
  function ws_get_plan_switch_url($plan_id) {
    $url = '';

    $user_subscriptions = wcs_get_users_subscriptions();

    if (ws_isset($user_subscriptions)) {
      foreach ($user_subscriptions as $subscription) {
        $subscription_order_id = $subscription->order->id;
        $subscription_key = WC_Subscriptions_Manager::get_subscription_key( $subscription_order_id, $plan_id);
        if ($subscription_key) {
          // get plan parent
          $plan_parent_id = wp_get_post_parent_id($plan_id);
          $url = WC_Subscriptions_Switcher::get_switch_url(
            $subscription_order_id,
            array('product_id' => $plan_parent_id),
            $subscription);
        }
      }
    }

    return $url;
  }