You are free to upgrade or downgrade at any time.
For monthly plans:
If you choose to upgrade, you will pay a pro-rated amount for the rest of the month. If you choose to downgrade, your next month subscription would be updated to the new selected plan.
For anual plans:
If you choose to upgrade, you will pay the annual pro-rated and due amount on a new bill. If you choose to downgrade, you'll have to wait until the end of your commitment.