Goto the below location:
/app/design/adminhtml/default/default/template/sales/order/view/tab/info.phtml
You will find the Payment Information there which is called on order details page. You can edit this according to your requirement and logic.
Also there are some more files where this information will shown like:
app/design/adminhtml/default/default/template/sales/order/creditmemo/create/form.phtml
app/design/adminhtml/default/default/template/sales/order/creditmemo/view/form.phtml
app/design/adminhtml/default/default/template/sales/order/invoice/create/form.phtml
app/design/adminhtml/default/default/template/sales/order/invoice/view/form.phtml
app/design/adminhtml/default/default/template/sales/order/shipment/create/form.phtml
app/design/adminhtml/default/default/template/sales/order/shipment/view/form.phtml
Hope this helps!