Transferring to a new phone
Can I transfer everything to a new phone before activation and still use old phone for a few days until activation is made?
iPhone XR, 16
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No. During the transfer it goes through an activation process. You may want to wait until you are 100% ready to switch over to the new device.
You can do the data transfer using phone to phone with Quickstart. There's no reason to keep using the old device at that point.
Use Quick Start to transfer data to a new iPhone or iPad - Apple Support