Change Apple ID on iPhone without causing 'damage' to apps/app store purchases and keeping app data
It is possible to change the Apple ID on an iPhone. You can do this by going to Settings > iTunes and App Store and logout with your current Apple ID. After that, you can login with the new Apple ID.

When you download/purchase an app from the App Store, it will be connected to the Apple ID that is used to do the download/purchase. This means that if you start using a different Apple ID to download/purchase new apps, there's no problem to update them.

As you mentioned, you have for example WhatsApp installed. If there's an update for WhatsApp in the App Store, you are still able to update it, but you have to use the Apple ID password of your "old" Apple ID.

On a side note, you can make a backup from your WhatsApp history (texts and media) in iCloud. After this, you can delete and reinstall WhatsApp and restore the backup.

I don't know if other apps that are important for you have the same functionality to create a backup to iCloud.