Thank you James.
I also found cr::site_call_to_change_mapping function in chg_site.spl in the samples directory. This has examples of how to change the data brought over from a CR.
I think I will still look into rebuilding this function using frm003, as there are a few bugs we've found with the Create Change Order button. For example, if a user takes longer than the ExclLockSeconds value to Initially save the Change Order the Change order reference may not be saved to the Incident.