Q: icloud account issues after switching to new iphone

Hi,

I've recently upgraded to the latest iPhone 6S plus and also at the same time changed my email address; now every so often I get a message popping up asking me to sign in to the old email address for icloud. however when I try to enter a valid password it then says that the apple id does not exist. I'm pretty sure that my icloud account is okay using my new email address but this old one keeps popping up and it's very annoying.

I'm sure it should be an easy fix as I'd just like to delete the old account but can't as even though I keep getting asked to enter the password it then says the apple ID account doesn't actually exist any more even though It's asking me to sign in to it and I'm just going around in circles.

Icloud information is unavailable on my phone presently so I'd just like this all cleared up with the one icloud account.

any help will be much appreciated before I book an appointment at the Apple Store.