I want to change the default Memcache serializer (Memcached::SERIALIZER_PHP) by replacing it by Igbinary (Memcached::SERIALIZER_IGBINARY) in a server that already contains caches.
If I make the change, I think that the Memcache server will attempt to unserialize the old caches with Igbinary and, therefore, it will crash.
Is that assumption correct? Is it necessary to perform a flush_all action in order to avoid errors?
That's correct. If you changethe Memcached serializer all existing entries are incompatible. You will need to perfom a flush_all to start fresh as you mentionned.