iPad's Apple ID did not reflect the change I made on and now I can't turn off Find my iPad So this is quite a headache already. I'll lay out what I've done chronologically:


Not knowing that the Apple ID is still in use on a device (iPad), I've changed the associated email address on the Apple ID website.
The Apple ID is now associated with the new email address
I completely forgot this ever took place.
The boss approaches me, asks for the password of the Apple ID. 
I forgot the password, so I reset.
No email came through
Checking Safari for saved password, found it.
Did not work
Found a similar password associated with a different email address
Suddenly remembered the change of ID


Now, for the questions:


Why didn't the iPad reflected the change I made regarding the email? This could've solved my problem, but it became stuck with basically a non-existent Apple ID.
Why can't I add the old email address as an alternative, or re-associate it with the Apple ID?
What do I do next? The iPad is associated with a non-existent Apple ID, and I can't turn off the Find my iPhone.


I hold you can help me with this issues.
I just went through this mess, almost the exact same steps (minus the boss asking). In my case I had forgotten about the iPad being logged in to the same account and did not turn off findMyiPhone on it before changing the appleId.

What worked for me finally, I found here apple's KB.

Log out of iMessage on the iPad, and the log back into iMessage making sure to use the NEW appleId email address.  Next go to settings and turn off FindMyiPhone, you will be prompted for a password and the old appleid will still be displayed in the request, use the password for new appleid that you just used to log in iMessage, and it will turn off find my iphone.

Once find my iPhone is off, you can log out the account completely, and update the email address.