Switching phones on a line with a paid off phone

My daughter has an iPhone SE and it’s super slow. She wants to use my old iPhone XR that is paid off. Currently, she has a $16 a month credit promo that she gets so that her $16 a month iPhone se charge comes out to $0. We still pay the $35 line fee. If we switch the phones, will we have to pay the $16 a month and not receive the credit (In addition to the $35 line fee)?

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