Unable to alter billing address during checkout

[steph.hope]

I've just noticed that an existing client seems to be unable to alter their billing address on the cart.php?a=checkout page. All of the inputs in the Billing Address section of the page are read-only and show the primary client's address. If the client is adding a new credit card they can add the credit card details but if the credit card's billing address is not the same as the client's address in the system the payment will fail.

If paying through the "Pay Now" button on the viewinvoice page they can successfully use different address details.

This is using the Six template and Stripe gateway, if that's relevant. Am I just not noticing some way of editing the billing address on the first image? This will be an issue whenever an existing client orders new services through the client area and wants to immediately pay using a credit card whose address doesn't match the client's address.