Allows the editing of the PayPal Standard Subscription ID even if it shouldn't be edited. Use at own risk.

functions.php

<?php // do not copy this line

/**
 * Allows the editing of the PayPal Standard Subscription ID even if it shouldn't be edited.
 * Use at own risk.
 */
function jp_allow_edit_of_pp_standard_sub_payment_20200416( $payment_meta, $subscription ) {

	// Make sure that what we're going to edit exists. 
	if ( isset( $payment_meta['paypal'] )
		&& isset( $payment_meta['paypal']['post_meta'] )
		&& isset( $payment_meta['paypal']['post_meta']['_paypal_subscription_id'] ) 
		) {

		// Set disabled to false to allow editing.
		$payment_meta['paypal']['post_meta']['_paypal_subscription_id']['disabled'] = false;
	}

	return $payment_meta;
}
add_filter( 'woocommerce_subscription_payment_meta', 'jp_allow_edit_of_pp_standard_sub_payment_20200416', 100,2 );