Of course itunes will delete the old data if you sync your iphone with itunes! You should put your iphone on itunes manual management, which lets you drag and drop what you want onto your iphone without syncing. do as the following: Connect your iphone to computer and launch itunes⟶ find iphone under the "Devices" category in the left panel⟶ check the "Summary" tab on the right⟶ click "Manually Manage" ⟶ "Apply". Well if you don’t wanna go through the troubles just get a iphone transfer to do all the backup thing.
as all we know , iTunes doesn’t allow you to sync contacts from iPhone to iPad directly，and bluetooth can do nothing to transfer too. Now I share my experience: you need a USB cable with the device can configure cantact in itunes .Using a third-party software is a quick way export the contacts from iPhone to computer and then restore the files