Although changing plans is free, you will most likely get your plan prorated and you will end up paying overage fees for the first part of the plan.
Say you have 200 minutes included in your current plan and you're at the middle of your billing period. You have used 199 minutes of those 200. Therefore, if you change your plan, you will have used 199/100 minute for the first half since the plan cost/minutes gets prorated when switching to/from a plan. Therefore you will pay 99 minutes of overage. Also, your new plan will get prorated the same way your first plan did!