convertObject.php

<?php

// Based on https://stackoverflow.com/questions/965611/forcing-access-to-php-incomplete-class-object-properties
function convertObject($object, string $from, string $to)
{
    // Serialize
    $serialized = serialize($object);

    // Replace the object-type
    $objectReplaced = preg_replace(
        '/O:\d+:"'.preg_quote($from).'"/',
        sprintf('O:%d:"%s"', strlen($to), $to),
        $serialized
    );

    // Replace the object properties
    $propertiesReplaced = preg_replace_callback(
        '/:\d+:"\0'.preg_quote($from).'\0([^"]+)"/',
        function (array $matches) use ($to): string {
            return sprintf(':%d:"%s"', strlen($to.$matches[1]) + 2, "\0".$to."\0".$matches[1]);
        },
        $objectReplaced
    );

    return unserialize($propertiesReplaced);
}

abstract class Animal
{
    private $name;

    public function __construct(string $name)
    {
        $this->name = $name;
    }
}

final class Duck extends Animal
{
    private $sound = 'Quack';
}

final class Cow extends Animal
{
    private $sound = 'Moo';
}

$who = convertObject(new Duck('John'), Duck::class, Cow::class);

print_r($who);

// Cow Object
// (
//     [sound:Cow:private] => Quack
//     [name:Animal:private] => John
// )

maybe a better api for that would have only $from and $to and get the class with get_class, so you can avoid covertObject(new Duck(), Moody::class, Cow::class)