My sister's iPhone had been irreversibly damaged. When she got a new one some time later, she had forgotten her Apple ID's password and created a new one. I managed to reset the password just yesterday, and she'd like to use the old account again, as she's bought many apps with it.
How would changing the account affect all data unrelated to the Apple ID on the phone, like call history and text messages? Are they somehow tied to the account, or can she simply change the account both on the phone and the computer and create a new backup without losing anything*?
* Except the apps from the new account, of course. No problem, she only downloaded free ones.
Best Answer
Any purchases that she made with the previous account are the biggest problem as they are non-transferable. This would include music and movies, as well as apps. Since you mentioned that she only downloaded free apps, it shouldn't be a monetary problem. Because updates will require you to login to the old account, it would probably be better to re-download the apps with the new account. That way updates are seamless and easy.
iMessage (or SMS and MMS for that matter) shouldn't be a problem at all. All the old threads and messages should stay in tact and be usable like nothing ever changed. Call history isn't something that iCloud, or an Apple ID has anything to do with, so the change will also be without consequence there.
Yes, change Apple IDs, create a new backup and you're good to go.
Side note: iPhone actually handles Apple ID changes very easily and smoothly. Some people have two Apple IDs in general - most by accident - and they will change back and forth for app/music/movies purchases.