They lose the amount of the last rate hike every month after May. So, if the last Feb rate hike would have hiked their bill by $5 and they went through Jan, they would have to pay $5/month extra for May-Jan, so perhaps somewhere around $35.
For me it means that I can downgrade when I want to without taking a chance on losing my annual billing. Looks like for now I will pay $5 more than I would have paid on the increased rates but will downgrade to the 200 package to save $10. So I come out ahead. I can always go back to the 250 package if I need to.