You can Upgrade (or Downgrade) your plan at any time.
If you Upgrade, your new plan will become effective at the same time you confirm the plan change, charging you the price of the new plan minus the previous plan that was in effect. The following month you will be charged the full price of the chosen plan.
You will keep your current plan and retain your remaining monthly contacts until the end of the current billing cycle. The following month, your rate will change to that of the lower plan, or to zero in the event that you have switched to the free plan.