Q: my serial number is replaced !!!

Talk to whoever you got the phone from. Return it and get your money back if possible.

That message indicates that the device was replace by Apple either in or out of warranty and not returned as it should have been, or that it was reported lost or stolen from Apple stock or in shipping.

It will be impossible to get ANY support or service from Apple for that device.

It's impossible to say. The only thing I can tell you for certain is that you will never be able to get service or support for that device from Apple.

Apple won't do anything. If it was reported lost or stolen in transit, however, it's possible that it's already on a blacklist. If so, you will not be able to use it on carriers that have it on their blacklist.