@Hugh Proctor My apologies for the delay! To prevent delays like this in the future, make sure the tags you place on the question is as close to the services you are referring to as possible.
There are some types of subscriptions which can have their offer type changed, and you can find more information Here.
Your best bet for this will be to create a support request for Azure Billing & Subscription to see if they can help with changing any accounts or subscriptions. They will also be able to give you specific instructions based upon your subscription and account types.