Apple ID is different to my iTunes & Apps ID
I have and iPhone and 2 iPads (one is iPad retina 4th generation, the other is iPad Air), I changed my Apple ID and it changed on all three devices, but my iTunes & Apps ID has changed correctly on one iPad and my iphone but not the other iPad - the retina. So I cannot download apps or update apps on this iPad.
When I go into Setting and then iTunes & Apps, my apple ID is showing my old Apple ID and is shaded out so I can't amend it. I was going to sign out, and sign back in but was worried that I would lose everything and won't be able to sign back in as it says all data will be removed from this iPad. Any advice, please?