Hi there, @OiMoose! Thank you for contacting us. In order to activate a new phone, you need to enter the serial number into your account and link a new SIM card to it. SIM cards are attached to a phone, so they cannot be used by another phone if it's already linked to a certain device.
You can activate your new phone here: https://myaccount.boostmobile.com/activations.html.