When changing to a higher speed plan, the new plan will come into effect immediately.
You’ll get credited a pro-rated amount for the lower speed plan you’ve already paid for, and get charged a pro-rated amount for being on the higher speed plan for the remainder of the month in your billing cycle.
When changing to a lower speed plan, the new plan will start from your next bill cycle.
You will be charged as per your current plan for the current month. From your next billing cycle you’ll be charged as per the new plan with the lower speed.