To be able to change payment type first you need to delete all current payments and re-settle ticket with correct payment types.
Implement this to cancel payments. Reopen ticket does nothing by itself. It is useful when used with “cancel payments” action.
Button appears gray because of the button mappings configuration. Take a breath, edit automation command that you’ve created for cancel payments, go to mappings page and clear “Enabled States” value completely. After this change button should appear functional.