Make a new version of the Grand Campaign scenario (or whichever one you want to play), rename it, and open it with a word processor. You will see eight abbreviations for the countries playable in the scenario (i.e. FRA for France, HAB for Austria, etc.) and information for each of those countries. Change one of the abbreviations to that of the minor power you want to play (VEN for Venice, HUN for Hungary, etc.). You don't need to change the country information. When you start the game, the new scenario will be available with the former minor power in place of whichever major power you got rid of.