The cards are always compatible. The difference is only in the fact that being the card which has registered to itself the 4.5 upgrade now present in your system, you will be able to use the new plugins that are basically the difference with ver. 4.0.
For example if you try to load the new stuff that is in version 4.5 without having the new card installed, you would be prompted with a message asking you for the card to register the keys to. Obviously you haven't installed it yet, so the load would fail. But when your second card will be installed, the new keys that you have been provided with 4.5 for the new card will work and you will be able to run them.
In Scope system the plugins and the software are registered to the single cards which act as a sort of "dongle" to enable them. As long as a card with a certain key is present in the system, you will be able to load the plugin. You could have different keys spread to different cards, but as long as these are present you can load all the plugins.
