Using a script to import records using Update and Matching Names

Forgive me if this is a dumb question, but I am new to FileMaker.  I want to automate the process of users importing an "update" fmp12 file into an existing database.  Sometimes this update file will include a container field of images, other times (to keep the file size down) this field is omitted from the update file, so I don't think I can use Custom Order or Last Order because of this variation in table structure (and the user will not necessarily know which structure it is).    I had expected "Matching Name" would cope with this, but when I am creating the script, when I select the blue Match Field option (which I have to do because I want to Update, not Add) the "Matching Names" toggles to "Custom Order", and if I set it back to "Matching Names" the blue Match Field setting reverts to green Import.  Is there a solution to this (or am I doing something wrong)?

Sorry, just realised I should probably have posted this in a different topic (Script Workspace and Script Triggers?) but don't know how to move it!