Mistake or not, if you change your plan, your old plan terminates and you lose any remaining days. The new plan does not start until you pay for it. You need to call and explain the mistake and hope you get a sympathetic representative who will restore your plan.
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.