Videos
I think most people worry about this when they stop sharing an iPad. I used to hear these concerns all the time at the AASP I worked at. Let's address each issue in turn.
- Apps will still remain associated to your mom's Apple ID, but they won't disappear. If you sign out of your mom's account and sign in with the new one, the apps stay exactly where they are. The only snag is, when it comes time to update, you will be asked for your mom's Apple ID and password to update them.
- None of the data saved in apps will disappear when you switch an Apple ID as it is all saved locally on the device (except in the case of online games such as Clash of Clans etc, but they usually require you to log in with a 3rd Party service such as Facebook which then stores your save data instead).
- Unfortunately, the Game Centre achievements are linked to your mom's Apple ID and if you sign out of hers and in with another, those achievements will disappear. Often, games feature a local achievement board which will remain on the device, and there is a chance it would sync to the new Apple ID, but in most cases I'm afraid you'll have to get those achievements again. But hey, look at it this way, that old game you haven't played in months? You can have some more fun with it again trying to set your old high score! ;)
You can have a look at another post of mine here where I talk about backing up and restoring save games on an iPad when switching users. You can use the information in there to backup your saves, then delete the games and re-download them with the new Apple ID to circumvent the annoying need for your mom's Apple ID whenever they need to be updated.
Obviously, any purchased apps and games would need to be re-purchased if you do this, so I would recommend your mom enable Family Sharing on her Apple ID and add the new Apple ID to the family so you can access her purchases without paying for them again.
1. How will app ownership be affected by this change? Will apps disappear because the original purchaser is no longer logged in on the iPad?
Apps won't disappear. You can combine apps on the same device from different Apple ID's. Just switch Apple ID in the App Store at the bottom of the screen and download an app. The only downside is having to type different passwords when updates are available.
2. Will saved progress disappear when a new Apple ID takes over the iPad? Or does progress remain saved on the iPad hard drive?
Progress remains. Game Center achievements might not, though they are in the cloud anyway. If you want to take the app data from the old Apple ID to a different device, make sure iCloud is on with app data enabled.
3. What will happen to achievements in Gamecenter? Will they update to where they were when the new Apple ID launches a game that has already been there for years?
That depends on the game. Often they won't, as they're linked to the other Apple ID and the game mostly won't check if past achievements have been entered into Game Center. The achievements will remain on the old Apple ID, as stated in answer #1.
Summary: app data is often stored in iCloud and always on the device, so don't worry about losing progress. You might lose achievements, though they remain on the old Apple ID and as they don't align to your game progress anymore it will be a bit messy.
What part of the iPad do you want to add a new Apple ID to? There are several different places to enter an Apple ID.
Messages
Settings > Messages > Receive at:
App Store
Settings > Store > Apple ID:
Alternatively, you can go to the App Store, click on "Top Charts" at the bottom. Scroll to the bottom and you can sign out and enter a new Apple ID.
iCloud
Go to Settings > iCloud > Account:
You don't say which Apple ID you want to change. The iTunes Store, iCloud and Messages all use Apple ID, and they can all be different. But yes, you can change, one, or all, according to your needs. Most of these changes are made in various Settings on the iPad.