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Call Customer Service, and let them know when you want it to be effective when you order the change. I would recommend at the start of a billing cycle - either the current one (to cover any overages you may have NOW) or the next one, to cover any overages you anticipate. If you are already over, then backdate it!
This option is SUPPOSED to be available on the website, but call customer service to be sure - the option for the effective date is either not available or well hidden.
