I changed my Plan today after my ten days plan got expired. And got charged $ 28.25 and again $ 125.43 same time today
Being prepaid you're paying for the plan ahead of time, if you change the plan before expiration you're cutting the plan you've purchased short.
So if you selected the $25 plan it came to $28.25 after tax and then even if you immediately went and changed the plan to the $111 plan you're then paying for that new plan in advanced and are charged $111 plus tax ($125.43).
Actually, I changed my new plan few days after the expiry of the old plan which is today. I purchased my new plan as 90 days one, but I got charged twice