Yes, you can! You just need to go to "My Account" page then in the lower left click on the "upgrade my plan" link (you can upgrade and downgrade) and choose a different plan. The change will take effect immediately.
Note: You should use all email credits on your current month's plan and ensure that your balance is listed a "0 credits" before making this update, otherwise the credits will be forfeited. This is why we created monthly plans - so you can change to the most suitable plan for you!