Port in 2 lines from T-Mobile, then move the new devices with promo to existing 2 lines

So I have two lines now, with paid off phones on them. I want to port in 2 lines from T-Mobile, and get a device promo on both. When they arrive and I go through activation, can I simply swap the new numbers to an existing unlocked phone, place the new devices on my existing lines, and keep all promotional credits I got when I ported in?

See this thread to see exactly what I mean: https://forums.xfinity.com/conversations/devices/switching-number-associated-with-a-phone-purchased-on-a-promotion/66f6d719a6879c6c7ca90f0f

I just want to make sure this will still work. Thanks everyone.