Struggled with the same issue for over a week, but the solution was simple when updating or purchasing on Itunes:
First reset the password on the newer ID. Then reset the password on the old ID. I made the passwords the same. That way whether I've purchased items with the old or new username, I can enter the same password and my problems is finally solved.
If you don't know how to reset your password, once you enter your password incorrectly a few times, it'll ask you if you want to reset either by e-mail verification or answering security questions. You can pick either and reset.
My mistake in the first place was that I kept changing the new ID but it wasn't giving me access to items purchased on old ID
🙂