What does changing my IP address on my Mac effect?

If your Mac is connected to a router in your home/business then the router typically supplies your Mac with an IP address which it gets from your service provider (ISP). This is done through a protocol called DHCP (Dynamic Host Control Protocol) and the user has no control over it. Changing your IP address arbitrarily in your Mac will cause you no end of trouble. You can have what’s called a static IP address but your ISP has to support it and give a range of IP addresses. There is only a certain range of IP addresses that each provider has. You can ask your router to give you a new IP address by what’s called “renewing your lease” but there’s no guarantee it won’t give you back the same one.

So I guess the question to ask is why do you think you need to change your IP address?

Why do you want to change it? You don't change IP like selecting a different color drapery for the windows. Do you know you even can change IP? Many ISPs assign an IP to your service.