If you are familiar with manipulating XML, you can use an XML editor and copy/paste the relative formulas from one xmod to another. Or you can open the model in one instance of Monarch and the new model in another instance and copy/paste between the two, however if there are a lot of formulas this will be time consuming over directly accessing the XML.
If others have come up with alternative methods, I'd be interested in hearing about them too!
If the major part of the complexity of your model is related to the calculated fields you could use the original model with the new format ( I assume it is a text file rather than a database when I suggest this) and then alter the templates to suit. That may be less work than creating a new model and adding the fields. Use a copy of the model file of course.
The traps and samples can be adjusted on screen and the fields and positions adjusted using the Field List which would then retain the original field names for you.
Otherwise cut and paste as Nick suggested. If the xml edit route works try that as well, it's not something I have attempted.
I don't suppose there is any chance you had version 7 and the WorkStore utility is there? If you did there may be other possibilities.