@Doug Bulleit Thanks for reaching out.
Your credits cannot be used to offset past charges.
However, if you would like to make changes going forward, please go to the Azure portal and create a support ticket for subscription management including the details of both accounts. team will walk you through a verification process that ensures approval from both accounts for the change as well as any technical implications this may have on your existing environment.
Hope the above information helps, please let us know if you have any further questions. Thanks