You can upgrade or downgrade between premium plans at any time by going to your profile. and clicking on the link "Click here to change your plan," which is located under your plan indicator.

Note: You can not downgrade from a premium plan (Solo, Band, Company) to the free plan (Catalog), due to the fact that registrations are not available in the free plan. If you no longer want the ability to submit registrations and get ISRCs, you can cancel your account.