Understand the contract. Usually grandfather clauses come into play when negotiating a new contract. Employees need to know their rights. For example, if you are an employee and you need certain requirements for a job and the requirements change before you are eligible, you need to check if there is a grandfather clause. If you could substitute experience for education before and now you can't, you need to find out if a grandfather clause exists. This can make a huge difference if you are eligible for a promotion.
Know the law--zoning laws also use grandfather clauses. You business may be protected by a grandfather clause. It is possible to rezone a commercial area to residential; however, the businesses that are in the area already could stay due to a grandfather clause. However, if the business owner decides to sell, he may not be able to sell to another business. The wording of the grandfather clause will tell him who he can sell his business too.
Look at the rules in the sports you play. In the major leagues, rules change to protect the players. For example, in the National Hockey League there is a rule that all players must wear helmets starting in 1979. However, there was a grandfather rule that let players who signed a professional contract before this clause went into affect not have to wear a helmet.