It's a bit of a process but we had this situation (See steps below). Husband owned a iPhone 4, switched to Galaxy 4S, wife still has iPhone 4S. Galaxy could not receive messages from iPhones because iPhones still reading as iMessage rather than text.
Steps we did to fix it:
1- Changed username and password for AppleID (done through the support settings).
2- Logged into iCloud, deleted off iPhone from account as a device (the old phone no longer in use).
3- Logged into iTunes, deleted the iPhone off as a device (this seemed to be THE ONE!). Note: It took about 15-20 minutes before the message would send as a text to the Galaxy after deleting the old iPhone device in iTunes.
4- Did turn off the iMessage (we still had the old phone). We also turned off all types of phone and message related services in the settings. However, I do not think you need to do this.
Steps 1-3 were done on the computer online. Not on the device.
5- Reboot all phone devices.
I also did delete the contact, but it doesn't really matter with the contact stuff. I added it new - but it was still showing in my address book as iMessage but when I went to send the message it showed up correctly as a text and was then received.
Good luck. I read on another forum also that you can contact your carrier and they might be able to do something within the network to help you get it resolved.