Downgrade from one Subscription to a less volumed one without losing any remaining credits

Hi, I would like to downgrade from one Subscription to a less volumed one without losing any remaining credits OR pay just for the relative use (as the auto renewal was done today). I don't need all 250mil, i can succeed with 128 mil, but i don't want to pay twice. Once for the 250mil and the second for the 128. Thanks