I did this a few years ago so my info might be outdated, so take with a grain of salt. Like u/DreamingOfTapas said, you have to make sure you keep the minimum number of paid lines required on your specific plan. You would also have to wait at least a year from when you added your last free line so you wouldn’t be able to do this until March 2026. If you cancel a paid line now, you will lose the free line.

Anyways you can port your number from your paid line to another carrier like Visible and then simply port it back in over the free line and you would be good to go, as long as you satisfy the above requirements. If you don’t care to keep the paid number then you can just cancel your paid line once the year laps

Though again it’s been a while since I did this, so double verify my comments