No, when you switch your plan, your plan will continue on the existing membership until the end of the current billing cycle. Until the end of the billing cycle you will be able to change your mind about switching.
E.g. My monthly plan renews automatically on 9th September and I decide to switch to the yearly plan on 2nd September. I will have until 9th September to change my mind and stay on the monthly plan. On 9th September I will switch from monthly to yearly and be charged for the yearly plan.