Copyright?

Given the fact just about every game is based on some game that has gone before and many clones have appeared. What differences would have to be made to graphics or gameplay to avoid the wrath of the original programmer/company?

For example, a person wanted to produce a commercial Pacman-like game (no I'm not writing one) and just changed the graphics - would that be ok? By how much would the graphics have to change? What about gameplay changes and storyline?

Thanks, Steve.