Hi,
My name is Paul. I have been terrible upset with the new iOS. I actually feel a bit cheated, any way here is the solution to your problem.
It appears that for some reason the upgrade affects your apple ID. To solve this problem you need to log out with your current ID on your phone and create a new one. All the verification etc will be sent via email.
Then turn off the phone, and re-boot. Go to settings and enter your new apple id into settings.
Now when you go to messages in settings you should be able to enable your imessage feature. It`s a pain, but the only way I have been able to sort out the problem.
If not let me know as I have another idea but it is more labourious
Cheers, and good luck.
PS you`ll also have to sign out of itunes on your computer too, and then sign in withthe new id.