Hi TerryB,
First off, Full Trust is a bad practice. 95% of forms out there do not need to be Full Trust. Why does yours? Better have a good reason, otherwise my answer is to remove that. :)
OK, so you really need FT... and you have discovered that URN based forms can be made Full Trust. Next step is to actually create them. RegForm was the tool for InfoPath 2003 to do this. You will need to run RegForm to create an MSI that will install the URN-based form on your client computers. The form will not live on ShP (MOSS 2003) but rather on the client computers where it is installed. You can't create an FT URN-based solution by just changing the name. You need to create an MSI installer to do it (or register the form manually via the registry, i.e. registering the form).
Good luck removing your need for an FT form... :)