When changing your plan through Self Serve, the changes take effect immediately so if you are within your billing cycle when you make the changes you will have proration on your next bill. (meaning full month in advance of new plan and a prorated amout from what you used of the old plan and prorated amount of your new plan from the date changed to your billing cycle close date)

Please note when you are changing your plan mid cycle, your minutes are also prorated which means you will not be entitled to the full allowed minutes that comes with the plan chosen!