Preserve app data when moving to new Apple ID (complication: can't log in to old Apple ID) I need to set up a new phone with a new Apple ID for a friend. We're facing two technical problems: 


We need to preserve some data from the old phone (specifically, the WhatsApp chat log). 
We don't know the password to the Apple ID the old phone is linked to.


Current situation: We have the old phone (associated with the old account), and we have successfully updated it to the latest iOS version and backed it up via iTunes. The old phone does not have iCloud services/backup activated. 

We have the new phone (factory reset), updated to the same iOS version (11.2.2).

I was able to successfully restore the backup to the new phone. However, in order to install the apps (and thus access their data) from the old phone, they need to be downloaded with the old Apple ID, for which we don't have the password (and no way of resetting it either).

If I bind the phone to the new Apple ID, I have to delete and reinstall WhatsApp, so the chat logs will be gone. Is there any way to back them up and restore them to the new ID (the phone number/SIM card remains the same)?
You could try backing up the old iPhone's WhatsApp chats to iCloud. This is a feature of WhatsApp and would only backup and recover WhatsApp Data. This is different from the 'normal' iCloud backup, which backups App Data.

The WhatsApp feature is described here: https://faq.whatsapp.com/en/iphone/20888066/#backup

To backup the chats to iCloud:


  To make a manual backup of your chats at any time, go to WhatsApp Settings > Chats > Chat Backup and tap Back Up Now.
  This will back up your chats and media to your iCloud account; you can choose to include or exclude videos from the backup. 


To recover the chat history on the other iPhone:


  To recover your chat history from an iCloud backup, first verify that an iCloud backup exists in WhatsApp Settings > Chats > Chat Backup. If you can see when the last backup was performed, delete and reinstall the app from the App Store. After verifying your phone number, follow the on-screen prompts to restore your chat history. The same iCloud backup requirements apply for iCloud restore. In addition, the phone number used for backing up and restoring must be the same. 


This should work to manually backup WhatsApp data to iCloud.