When you order a plan upgrade, you will be credited for your current plan and charged for your new plan.
The credit will be for the unused portion of the old service period. The charge will be the new plan's cost for a prorated period, based on the account's monthly billing date.
In other words, a new plan on the 1-Year billing period will be charged for 1 year, rounded down to the nearest monthly billing date for a charge of 11.x months.
Any credit from the old plan will be applied to the new plan's charge.