Hi mbd-uk,
As long as the quantity of distribution circuits are applicable to the licence you have, the project will be opened by ElectricalOM.
Let's say there is a project done by Ultimate version which has 10 distribution circuits. This project can be opened by Ultimate, Advanced, and Professional, but not by Limited as this allows for 4 distribution circuits.
When it comes to add-on modules; if you have the module, then, you are good to go, if you do not have the add-on, then, you will loose the data associated with the add-on module.