Copyrights question

If I take an old game that some company did and rewrite the rules but use the original units is that considered still under their control for denying vassal play?

Or is it considered a new game? Secondly since denying me the use of a tool to play a game I own seems to be legal, isn’t that like saying you can’t use a table when you play our game?

Once I read somewhere that in the USA it’s allowed to copy the mechanics of a game, but using the original artwork is not allowed. Not sure if I maybe misunderstood something, and I have no idea what the laws in other countries say about this.