Definitely!
For plan upgrades: Upgrading from a lower individual data plan to a higher individual data plan can be done any time during the billing cycle. For example, if you are on the Mini, you can immediately upgrade to the Midi.
Upgrading to the Multi shared 5GB plan takes effect during the start of the next billing cycle. Upgrading from the Multi plan to a larger, individual data plan 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.