I experienced the same issue you did, iMessages only using the same account as my iCloud account. I was able to apply the same method as described above with the following changes. Since the iPhone opens the Message app without iMessages being enabled, you have to change access iforgot.apple.com through the settings app. I did the following:
1. Open Settings-->Messages
2. Turn off iMessage
3. Close the Messages app via the multitasking panel
4. Go back into Settings-->Messages and enable iMessage
5. Go down to Send and Receive and click line to enter
6. At this point you should see your phone number and blue text asking you to use your Apple ID...click the blue text
7. You will see a dialog that asks you for the password for your iCloud account but no way to change the iCloud account
8. Click on the ? box in the password field
9. It will take you to the iForgot.appleid.com page
10. Enter the email account you want to tie to your iMessage
11. Change your password
12. Cancel out and the account will now be available within imessage automatically.
Some of these steps may not be required, but this is what I did and now have the AppleID I want to use in iMessage that is the same as before iOS 11.
Hope this helps.
c