You can easily change your monthly plan whenever you want. Just visit the Switch Plan page when you're logged from the main account settings menu.
If you're downgrading then you'll retain access to your current plan until your next billing date. If you're upgrading it will upgrade you immediately and you'll be charged a pro-rata rate on your next billing date.
If you only need a higher usage plan for a short time during busier periods you can upgrade, then downgrade again right away - you'll keep the higher use plan for the remainder of your current billing period.