To the original poster: Don't fret. If for some reason they change the core rules and start printing the Clue restriction on the physical rulebooks that are included in the boardgame being sold, it would be accompanied by a change in artwork, an alternately numbered edition of the game, and lots of other changes to the rules (including all of the eratta published online to date).

In other words, if one of your friend spills Cream Soda all over your game in a few years and you have to go out and buy a new one to replace it, you're not going to be surprised by a rules change. If a friend loves your game and goes out and buys his own box, he's not going to get a different set of rules from you.

Is either of these the situation you're worried about?