there's a bug in the system
when the status of a subscription moves to disabled, say because the spending limit has been reached, it is not possible to 'manage payment methods' since this hyper-link goes away.
so, resolving the disabled status to turn the machines back on requires a support call in order to:
1) I create a payg account, attaching new credit card details
2) MS support transfers the credit card info to the suspended subscription
3) MS support then cancels the payg subscription
4) I remove the spending limit
it is ridiculous that this cannot be solved by the subscription owner themselves without having to involve waiting for MS support to resolve the issue.