I have the same problem you report, Stephen. I have been assigned a previously used iPad for work, and after using the Reset for all settings, iMessage is still trying to use the old Apple ID. There is no way to change it, no option to Edit.
After searching online I've tried the following, all to no avail:
- Signing out of Store and signing back in.
- Using Reset to reset all settings.
- Typing the wrong password in repeatedly in hopes Apple gives me a chance to try something different (no luck after 8 tries).
I can't use the old email because it's not mine.