You can open models developed with earlier versions of MLDesigner, you can view them like any other model in the Model Editor but before you can run a simulation you have to update these models.
Whenever you open a model in MLDesigner, the XML file (.mml) is parsed by the version mapper. If the version number is higher than the actual number of your installed MLDesigner version, the model cannot be opened. If the version number is lower than the actual number of your installed version of MLDesigner, the model is opened but it will not be touched unless you explicitly save it or run a version update.
If the old models are located in a library, it is highly recommended to run the version update instead of saving the models one by one. The version update can be invoked via the Tree View, right-click on the library item and choose Update Version.
Aside from increasing the version number for each model, the version update maps model instances to compatibility models if they exist. MLDesigner provides compatibility models for primitives and modules in case their behavior has changed in this or past versions of MLDesigner. These compatibility models are located at their respective domain library (for example, MLD Libraries→DE Domain→Compatibility).
Besides behavioral changes, it is also possible that interfaces of primitives and modules have changed. This affects names of ports, parameters and shared elements (that is, memories, events and resources). These names are changed in each model instance, which corresponds to a model with a changed interface, by the version update.
To account for changes made to the MLDesigner kernel, the version update also includes updates to the primitive source files. There is, however, no guarantee that the resulting primitive source code is 100% compatible with the current version of MLDesigner. This is especially the case when experimental domains are involved. You might also run into trouble when the primitive which will be updated was developed on a different platform than the one you are currently working on.
After the version update is done you should examine the Log window closely. There, you will find information about the exact changes that were made.