Current promos for existing customers upgrade, devices, etc.

Existing Tmobile customer w/3 lines. Financing one lines phone. I’d like to get my daughter a newer phone, as she has a much older device (iPhone 10/11) I think. Phones very glitchy, and just old. So she is on another Tmobile plan w/her grandmother. I’m wondering if I can trade her phone in under my service for whatever promo they’re currently running for this. I also wouldn’t mind upgrading my phone (iPhone 15 pro max) paid off, and maybe give my phone to my daughter, and trade her older phone in towards an upgrade. Just trying to figure out what makes the most sense, both financially and ideally. Any suggestions?