Q: If I change the Apple ID
... for iTunes, iBooks, and the App Store on my iPad, will the items (especially Apps) that I bought or downloaded using my old ID still be available?
Posted on Apr 11, 2015 6:14 AM
by King_Penguin,Apple recommended
iTunes
If they are already on your iPad then they won't be deleted if you log in with a different account - they will all remain tied to the old account, so for example for apps only the old account can download updates to its apps and make in-app purchases in them (so for IAPs you may need to log back into the old account to do them).
Using iTunes Match, automatic downloads or redownloading an account's past purchases may tie your iPad to that account for 90 days : Manage your associated devices in iTunes - Apple Support
Posted on Apr 11, 2015 7:33 AM