Change user's Membership Level when a product is purchased in Easy Digital Downloads

change-level-edd.php

<?php
/**
 * This recipe will allow you to assign a membership level to members when purchasing an EDD product. 
 * You can do this one of two ways: 
 *
 * OPTION 1: Product ID and Membership Level
 * $product_levels = array(
 		27 => 1, //27 is the EDD product ID. 1 is the membership level they get when purchasing product 27
 	);
 *
 * OPTION 2: Product ID with custom Membership Level Array - Change parameters for that level on the fly such as an expiration date. 
 * $product_levels = array(
 		37 => array(
			'membership_id'		=> 2, // integer - Required
			'user_id'		=> $payment_meta['user_info']['id'],
			'code_id'		=> '0', //Discount code
			'initial_payment' 	=> '0', // float (string)
			'billing_amount' 	=> '0', // float (string)
			'cycle_number' 		=> '0', // integer
			'cycle_period' 		=> '', // string (enum)
			'billing_limit' 	=> '0', // integer
			'trial_amount' 		=> '0', // float (string)
			'trial_limit' 		=> '0', // integer
			'startdate' 		=> current_time( 'mysql' ), // string (date)
			'enddate' 		=> '2021-12-31' // string (date)
 		),
 	);
 *
 */
function my_pmpro_change_level_edd_update_payment_status( $payment_id, $new_status, $old_status ) {

 	// Basic payment meta.
	$payment_meta = edd_get_payment_meta( $payment_id );


 	// OPTION 1: Set the relationship between EDD product ID and level ID.
 	$product_levels = array(
 		20 => 1, //27 is the EDD product ID. 1 is the membership level they get when purchasing product 27
 	);

	$user_id = $payment_meta['user_info']['id'];

	// Allowed statuses to let people keep their level.
	if ( $new_status == 'complete' || $new_status == 'publish' || $new_status == 'partially_refunded' ) {
		// Order is completed or partially refunded. Make sure the person gets/keeps their level.
		foreach ( $payment_meta['cart_details'] as $key => $item ) {
			if ( ! empty( $product_levels[$item['id']] ) && ! pmpro_hasMembershipLevel( $product_levels[$item['id']], $payment_meta['user_info']['id'] ) ) {
				pmpro_changeMembershipLevel( $product_levels[$item['id']], $payment_meta['user_info']['id'] ); // If a user buys the product with ID of 27, change their level to 1.
			}
		}
	} else {
		foreach ( $payment_meta['cart_details'] as $key => $item ) {
			if ( ! empty( $product_levels[$item['id']] ) ) {
				pmpro_cancelMembershipLevel( $product_levels[$item['id']], $payment_meta['user_info']['id'] ); // If a user buys the product with ID of 27, change their level to 1.
			}
		}
	}	
}
add_action( 'edd_update_payment_status', 'my_pmpro_change_level_edd_update_payment_status', 10, 3 );

See full guide here (with revised code): https://www.paidmembershipspro.com/give-customers-membership-level-after-edd-purchase/