It think is something you might want to contact customer service and ask.
I am unsure but I don't think it will allow a plan change until after activation. I think only changes can be made after activation. If you signed up for the base plan and want one of the new plus plans just activate and then upgrade, you will be prorated the difference for the month. If you signed up with the plus plan and wanted the base plan you should talk to customer service and ask if they can make the change prior to activating.
