Do I really have to change my slogan?

I think you have an error in your question. The company likely has a registered trademark in the phrase, not a copyright. The Copyright Office has a very low threshold requirement of creativity to get a copyright issued, but I'm not aware that they've ever awarded a copyright on a three word phrase. Do you have to change your slogan? You are not 'required' to change your slogan until a court issues an injunction requiring you to change your slogan. Until that time you can defend your right to use your slogan. However, getting that injunction will be expensive for the trademark owner and also for you, prospectively a five-figure legal bill for each of you. To get the injunction, the trademark owner will need to show the court that consumers are likely to be confused by the slogan into believing your company is related to their company. If they can show a likelihood of confusion, they could get the injunction and collect monetary damages from you. What you really need to ask is, "is the slogan worth it?" Do you really get enough business from the slogan that it is worth the hassle and the risk? Relatively few of these cases every reach a verdict because the more practical decision is to modify the slogan and avoid the expense. Good luck