How to Deregister iMessage
It's Now So Easy to Leave iPhone For Android
Once upon a time, the biggest pain of switching from iPhone to any other phone out there was having your texts forever trapped in iMessage. Finally, Apple has made it easy to move to a different smartphone without missing a single text: by deregistering iMesage.
If you still have your iPhone, go to Settings > Messages and turn iMessage off. If you have already parted with your iPhone, visit Apple's Deregister iMessage page, where you'll enter the phone number to untether from iMessage. Smartphone freedom awaits!