RE: Converting Order to Invoice using eConnect problem
You are asking if it is possible to convert a pre-existing order into an invoice using econnect, resulting in the same behaviour as if you had Transferred the order to invoice using the Transfer button in the user interface.
There is no native method for this operation in econnect that I am aware of, the econnect method simply does not exist.
By merely changing the numbers in econnect for those fields, all you will create is a copy of the sales order as a new invoice document (assuming you are populating the econnect doc with the order information). That invoice will point to the original (due to you changing those ORIG fields). The business logic to CONVERT the order will not have been run "inside" GP, hence all the goodness that normally occurs, like skimming off the quantities to invoice, and pushing any orders will all lines invoiced to history will not happen.
You will need some modification or GP scheduled automation software to accomplish this or a more sophisticated approach in your own code.
For example, MC2 and Envisage Software both have skills and products in this nature of GP automation.