Modify checkout message on restricted donation level for members switching

my-pmpro-replace-checkout-member-donation-message.php

<?php
/**
 * Modify the checkout message based on the "Donations-Only Level" setting.
 *
 * Ensures that existing members are only prevented from checking out with the donation-only level
 * if the "Donations-Only Level" option is checked in the membership level settings.
 */

/**
 *
 * @param string $content The original checkout message.
 * @return string The modified message if conditions are met.
 */
function my_pmpro_modify_donation_checkout_message( $content ) {
    global $pmpro_level, $current_user;

    // Ensure PMPro functions exist and the level is set.
    if ( ! function_exists( 'pmprodon_is_donations_only' ) || empty( $pmpro_level ) || empty( $pmpro_level->id ) ) {
        return $content;
    }

    // Ensure we are checking out for a donations-only level.
    if ( ! pmprodon_is_donations_only( $pmpro_level->id ) ) {
        return $content; // Exit if this level is not donations-only.
    }

    // Ensure user already has an active membership before modifying the message.
    if ( ! empty( $current_user->membership_level ) ) {
        // Get the current membership level name.
        $current_level_name = '<strong>' . esc_html( $current_user->membership_level->name ) . '</strong>';

        // Modify the checkout message.
        $content = preg_replace(
            '/Your current membership level of .*? will be removed when you complete your purchase\./', 
            'You cannot checkout with this donation level because you already have the ' . $current_level_name . ' membership level.', 
            $content
        );
    }

    return $content;
}
add_filter( 'the_content', 'my_pmpro_modify_donation_checkout_message', 20 );