The city of PyeongChang decided to change the spelling of its name before the Winter Olympics because the capital city of North Korea is Pyongyang, and the Olympic committee did not want anyone to travel to the wrong country or be confused in any way.
There are very strict travel regulations to and from North Korea so if there were a lot of people in the country unexpectedly there would be a potential for a major international incident. The city will likely go back to the original spelling after the Olympic Games are over.
PyeongChang is a county in the Gangwon province of South Korea. PyeongChang was the winning bidder for the 2018 Olympics. The reason Pyeongchang capitalized its 'C' a month before the games because people were confusing it with Pyongyang, the capital of North Korea. There have been cases that people had traveled to Pyongyang instead of PyeongChang and they wanted to avoid this.
Another change is the addition of the letter 'E' as the county was originally 'Pyongchang'. This decision was orchestrated by the Olympic committee in conjunction with the media. The city also sits 50 miles from the DMZ and there was potential for a disastrous mistake.