Definitely!
For plan upgrades: Upgrading from a lower shared plan to a higher shared plan can be done any time during the billing cycle. For example, if you are on the Basic 1GB plan, you can immediately upgrade to the Moderate 3GB plan.
Upgrading to the All-In Unlimited 20GB plan only takes effect during the start of the next billing cycle.
For plan downgrades: At any time during the billing cycle, you can request to downgrade your plan from a higher plan to a lower one. All downgrades take into effect at the start of your next billing cycle.