How to disable Stolen Device Protection on my iPhone before upgrading?

That is the purpose of Stolen Device Protection to prevent critical changes on your phone when it is not in a familiar location. It is optional and you can certainly turn it off if you wish. It would make no sense for any security measure to turn off the phone as you would want to protect any changes on your device when it is on. If you had your phone stolen and the thief had access, this would give you 1 hour to mark the phone as lost or erase the device before any changes could be made that would stop you from doing that.

About Stolen Device Protection for iPhone - Apple Support

Other than that your phone is already well protected with the Passcode that will automatically lock if too many failed guesses are made. You can continue to track, lock, and erase the device remotely and the thief will not be able to stop you unless they know the Passcode. The need for using Stolen Device Protection is your choice and frankly I do not use it. It is not turned on be default, so that is something you would need to explicitly turn on if you felt it would be necessary.