This change makes it a pretty attractive plan for very light users, or for backup/emergency use.
The per-unit rates aren't bad : compare Ultra's 3c/min, 1c/text, 3c/mb to Tello's PayGo at 1c/minutes/texts, 2c per mb. (And if you used all of it, the Ultra base plan includes units that Tello would charge $4 for.)
The payment scheme is a little awkward, and would require some user interaction. AutoPay charges $5, $10, or $20 monthly, with the $3 monthly fee deducted from that. With usage not exceeding the base units, you'll accumulate a balance over time. (Recharge rate is higher than monthly base cost.)
It looks like you can manually fill in $10, $15, $50 increments instead of relying on autopay, and let the balance draw down for both the $3/monthly charge and any overages.
Still a hassle that you can only set up a plan at 'select' T-Mobile stores. Does anyone know what the setup cost is?