Can I charge a different payment method on an existing trade?

There would not be any issue with initiating a second fund movement against a trade as long as the first payment has failed (returned). The payment method can be updated on the TransactAPI dashboard by navigating to the Trade ID hyperlink and selecting the Update Trade button. 

If the correct payment method is linked (for credit card and ACH) you will then be able to initiate the fund move from the same Trade Details page.