changing the field "formtitle" in web sales orders

We are quite new to the SuiteScripts. Until now, we have managed to modify the values of fields and it all works very well. However, we didn't manage to modify a field in the following case:

A sales order is generated from our webshop, default form used is: Custom Sales Order - invoice (ID=126). After the order has been created by our online customer, we would like to change the sales order form into the following form: Custom Sales order - cash sales (ID=127).

We are using the following postsourcing function:

function WebSOPostSourcing()
{
nlapiSetFieldValue('formtitle', 127);
}

But this function is not creating any change, the form remains unchanged. Does anyone know if it is possible to change a sales order form using Suite Scripts?