According to the article, it seems that it will be based on the fare type, not necessarily that you can "pay" to select a seat with the lowest fare. But the details have not been announced.
What I took away from the article is that for the lowest fare you get a seat assigned to you. If you want to select the seat, you pay. And they still plan to have boarding numbers. Don't know how those will be assigned.
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For your comfort, we’re introducing seat assignments with different options based on the experience you prefer. Our lowest fare offer will still come with two free checked bags,1 no change2 or cancel fees,3 travel credits that never expire, and a seat assigned to you prior to your day of travel.
If you prefer to select your seat when you book your flight, including seats with extra legroom, you will have a few options to purchase these benefits. Our boarding process will be similar to what it is today. You’ll board in order of position numbers and line up by signage displayed on stanchions within the gate area, so you’ll be on your way as quickly as possible.