The issue is that only retirement extensions can be approved locally, at the discretion of the IO - marriage extensions are sent up the line for sign off. Coupled with fact that you first have to evidence adherence to the last terms of approval (you can't) it forces you back to the agent.
I imagine that the only way you could do this is to let the current approval lapse and then start over again, on a marriage basis, with 400k in your bank.