How far does protection for copyright go?

I want to copyright a logo I made on my computer. The logo has a phrase in it. When I register the logo with the copyright office, how much does it protect my logo. If someone changes the font of the phrase can that person put a new copyright on it? Hypothetically speaking, let's say my phrase was "I love Steve" If someone made a small change to it and changed it to "I love John" can that person copyright that or it is a violation of my copyright?