Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
All the purchases made on her phone are tied to the Apple ID used to purchase them. Even if you change the Apple ID on her iPhone, she still need the old ID to update the apps downloaded using the old. If she want to update the Apps, then she have to delete them and redownload them using new ID. But the musics and movies downloaded to that phone will not be deleted when you change the Apple ID (unless the musics are stored on cloud). You will not be able to transfer these purchases to new Apple ID. Apple doesn't allow that.
There are mainly two IDs - Apple ID and iCloud ID. Apple ID is the one you use to download apps, musics, movies etc.
iCloud ID is the ID you use to sync contacts, calendars, backups etc between Apple devices etc.
You have to change both on her iPhone. So first create her new Apple ID from here --> https://appleid.apple.com/account
Then on her iPhone, go to Settings > iTunes & App Store. Tap your Apple ID and sign out. Then sign in to her account.
Then go to Settings > iCloud and you will see your iCloud ID there. Scroll down and sign out.
Then go to Settings > Messages > Send & Receive and sign out from your Apple ID in there. That is all you needed to do.
Also, if she want to get latest updates for the apps she downloaded using your Apple ID, then she has to delete those apps first and redownload them using her new Apple ID.
If you have any questions or would like any additional information, please don't hesitate to ask.