Does iCloud backup overwrite existing data? I got a new iPhone and since I had installed a lot of things over time I don't need/want anymore on my old one, I installed the new from scratch, downloading apps, adding the needed accounts, imported WhatsApp msgs via WhatsApp backup etc.
Now what's missing is e.g. my old SMS/iMessage and Signal history.

Since there's no export/import mechanism I tend to use iCloud Backup to get the missing pieces from the old to the new iPhone - and to save space and make it fast I removed everything I already have like music (can be synced later) and photos (already on my Mac in Lightroom) and most of the apps.

Question now: If I create a new iCloud backup on the old iPhone with only a small set of apps (thus focusing on SMS/iMessage) and restore it on the new iPhone, will this just add the missing pieces or also clean up the new device and remove everything that's not in the backup (and by that destroy everything I already set up)?

And if it would destroy existing data: is there any other way to get SMS/iMessages into the new device?
Restoring an iCloud Backup will cause you to lose existing data. As part of the procedure to restore an iCloud Backup, one chooses Settings → General → Reset → Erase All Content and Settings. Therefore your data on your existing device is gone.

It may be possible to perform a delta restore of an iTunes backup. Make sure both devices are running the same version of iOS, then…


Backup your current device (and your new device in case anything goes wrong) using iTunes.
Using iBackupBot, open the backup you made of your old device and find the messages data in the messages container in the backup.
Use iBackupBot's restore functionality to restore this data on to your new device.