Can't turn off Find my Iphone with old apple id password in order to delete account from my iphone

Iphone 4 - I'm trying to change the apple ID on my iphone to match my icloud (already changed apple id here) but it keeps asking me for the password linked to the old ID to turn off find my iphone, in order to delete the old apple ID account from the phone. I need to do this so I can sign in again with my new ID. This password of course won't work as the id has already been changed on icloud so the old email address/ID (now showing in the pop up box) doesn't exist! Any ideas?