Swapping SIM to old phone from phone on payment promotion

My mom and I were both eligible for an upgrade, and recently upgraded to the iPhone 12 with the $1/month promo (with statement credits).

My mom is a senior and having trouble adjusting to the full touch screen phone. She wants to switch her SIM back to her old iPhone SE for the time being, but still keep the 12. If she’s not using the 12 as her active phone, will we still receive the statement credits that make it only $1/month, or will we start getting charged the full amount?

I’ve seen conflicting information in similar posts, so I wanted to ask for my situation specifically. Thanks in advance for any help!