OneCare has a billing support option that would be able to help with this type of request.
In OneCare click "Get help with onecare" on the left then click on "Show me more topics" which will open a new window.
Here, click on "Get More Help" at the botom right corner of this new window then click "Get support".
Here, choose Billing Support and follow the instructions to submit an email/chat/phone case and they will be able to help you change from Visa to AMEX.
You may also be able to go to http://billing.microsoft.com and sign in with the same Windows Live ID that you used during installation, and there are options there to change your account details.
I hope this helps.