I work for T-Mobile corporate and we were always told if you did keep and switch you don’t qualify for device promotions until 90 days has passed.
The T-Mobile website right now is running a promotion where you can do BOTH at the same time.
This is what is on the FAQ page:
Can I keep my current phone, have T-Mobile pay it off and also take part in one of your phone promotions when I switch?
No, you can either choose to get reimbursed for your current phone’s remaining device balance via Keep & Switch when you bring your eligible phone and number with you or take part in one of our current phone promotions. Talk tour customer support team to see what makes the most sense for you!
But this is the temporary deal for switchers:
We’ll pay off your phone contract when you switch.
Break free from your 3-year contract with AT&T when you switch to a Go5G Next or Go5G Plus plan today. We’ll reimburse your remaining device balance and early termination fees, up to $650 per line, for up to two lines, via virtual prepaid Mastercard®.
Plus, switch today and get a new 5G device ON US: up to $830 when you trade in an eligible phone.