Yes, you can easily switch between the plans. Upgrade your plan is the same as if you had decided on the larger plan at the beginning of the current billing cycle. You will only be charged the difference to your current plan. So you can start with a RoboVoice S subscription, and if you notice that S is not enough, you can easily upgrade to M or L with immediate effect. You can also downgrade an existing plan, for example from L to M. However, a downgrade only takes effect at the end of the current plan and is effective for the next billing period.