How can I script a change from a GET to POST ?

As I await the client to be updated to do a POST instead of a GET, I would like to use fiddler to change it on the fly for me.

I have code in OnBeforeRequest that makes changes in parameter names, but I still need to convert the call from a GET to a POST and I cannot find any methods that will switch it for me.

 How can I switch a GET to a POST in Fiddler Script?