Is there a way to change the Apple ID on an iPhone without the user losing access to their data?

It is almost certain the phone is configured with an Apple ID, which might reveal the email for the original owner.

On the iPhone go to Settings, and select Apple ID (the top entry in the list of options, usually User's name). This should open to show at least the email address which identifies the Apple ID. Whether the original owner is still using it is open to debate, but it should be a start in the search.

Of course it is also possible the ID already points at the new user, but she simply didn't understand what you were asking about.... If that is the case it should be safe to force a password change to something she knows and will remember, and all the problems will be solved.

If you forgot your Apple ID password - Apple Support

嗨，1242区！很抱歉听到这个消息：<

但实际上，如果您已经登录了 苹果ID ，大部分数据仍然存储在iPhone中，而不是存储在icloud中。如果你没有订阅 iCloud ，它只有5GB，远远不足以容纳iPhone上的所有数据。

您可以尝试备份所有数据，然后创建一个新的Apple ID并登录。存储在iPhone中的数据不会消失，所以不用担心。

成功登录后，尝试打开 iCloud 的同步 .如果您希望可以保存数据并在任何其他Apple设备上进行检查，则可以选择订阅iCloud以获得更多存储空间。

这是苹果支持备份你的iPhone，看看如何备份你的iPhone，iPad和iPod触摸 - 苹果支持

Hi, Zone1242! Sorry to hear this: <

But actually, if you have logged into your Apple ID, most of your data is still stored in your iPhone instead of in iCloud. If you don't have a subscription to iCloud, it's only 5GB, which is nowhere near enough to hold all the data on your iPhone.

You can try to back up all your data, then create a new Apple ID and sign in. The data stored in your iPhone won't disappear, so don't worry.

Once you have successfully logged in, try turning on iCloud Sync If you want to be able to save your data and check it on any other Apple device, you can choose to subscribe to iCloud to get more storage.

Here is Apple Support about how to update:How to back up your iPhone, iPad, and iPod touch - Apple Support

Hope it could work

I hope she has been backing up all that data. If she has not been using iCloud – or any other cloud service – and her only copies of her files ("media that has been created over the last few years") are on that iPhone, she could lose all of her files in an instant.

She’s been using someone else’s ID? Yeah, that’s going to be an issue. First, if the previous owner did not remove their ID the device will be activation locked under their credentials. Only the owner will be able to unlock. Second, any items added to the device have been syncing to the previous owner; pictures, videos, purchased items, etc. in fact, any purchased items would have been charged to the other person. It seems likely that your friend does have an Apple ID given those syncing issues.

the best course of action is to factory reset the device, have the previous owner unlock it, and then set it up as new.