Work around for Easy Digital Downloads Payment History report for showing account username than guest after using EDD Guest Registration to convert a guest account to real.

functions.php

function dc_edd_get_payment_meta( $meta, $payment_id ) {
	if ( -1 == $meta['user_id'] ) {
		$user_id         = get_post_meta( $payment_id, '_edd_payment_user_id', true );
		$meta['user_id'] = $user_id;

		$user_info         = maybe_unserialize( $meta['user_info'] );
		$user_info['id']   = $user_id;
		$meta['user_info'] = serialize( $user_info );
	}

	return $meta;
}
add_filter( 'edd_get_payment_meta', 'dc_edd_get_payment_meta', 10, 2 );