First thing that comes to mind is to make sure iMessage is off and disabled before the switch.
Wish I had know about that before my wife made the switch. She was ready to toss her new HTC One Mini out the window. Another reason iHate iDiots who iNvented iOS and iPhones.
Update: If all else fails, call Apple Support (in US 800-694-7466). They are aware of this fiasco of screwing up texting for iMessage users whose contacts switch from iPhone to Android.
In a thread on an Apple forum, one poster claimed that changing your password on your Apple ID would reset the iMessage for the iPhone associated with your account to off. Another poster claimed it worked. I tried it yesterday morning.
In the evening after testing and a sender with an iPhone got a "not delivered" response to his text, I called Apple. Rep stated that the number was "challenged" (whatever that means) and that the problem should go away in 24 hours. Damn, I can buy a burger at McDonalds with my credit card and it shows up on my account immediately. It takes Apple 24 hours to fix their problem with the iMessage system? A rep can't push a key and delete the number from the system immediately?
iNcompetent. iNsane. iNfuriating!