I am looking to add my mom to our plan. If I add a new line I am able to get the A71 for "free". I understand that there will be a promotional credit credit for 24 months for this phone on the new line. If I decide to change the phone on her line to an iPhone X (immediately) will the credit still be in effect? We may also change out my wife's Note 8 for the A71 if we can do the swap for my mom.