I am going to cancel my second line, but I want to transfer that phone, which is newer, to my original phone number.
After you cancel your second line just reactivate it as a replacement for your first line. The number will transfer instantly and the phone you replace will be deactivated. You can cancel service on your second line in the Phones section of your account. That number will be lost.
It may be important to know which phones you are using. For example, are they both 3.0, 2.0 or a combination?.