Hi Vanhj01 and welcome to the forum.
Kruncher is correct as ever. You can use a V6 model directly and Monarch will 'convert' it internally to the xml form it requires for recent versions.
If you need to make changes to the model it will be saved as an .xmod file and not a V6 .mod file. There is no backward compatibility to V6 - the product and its underlying technology have moved a long way in the last 9 years.
As part of V10.5 the Monarch Utility has a feature to mass convert text based (.mod) models to the new .xmod format. Also it will convert project files and has several other features available.