Simplify the following text for a Grade 7 native English-speaking person.
Original: Holmes was trying to tie Rocky Marciano 's record of 49-0 as the Heavyweight champion , but it was Spinks who made history that night , winning a fifteen-round unanimous decision and becoming the first world Light Heavyweight champion ever to win the world Heavyweight title .
Simplified: It was the first time a light heavyweight champion had become heavyweight champion .