A subscription can be changed immediately or at the next term renewal. Immediate changes will bill the change right away. At term renewal changes will reflect the change on the first bill date invoice of the renewal term. It is not currently possible to make an immediate change to the subscription, but bill that change at term renewal. Please contact Recurly support if you would like to see this functionality added.
Most often we see that merchants prefer to make upgrades immediate and downgrades at renewal. Immediate upgrades allow you provide the customer the higher value product and collect the additional money right away. At renewal downgrades allow you to lock the customer into the agreed upon, and paid for, billing cycle, continuing to provide the customer with the higher value product until the next renewal.