Simplify the following text for a Grade 11 native English-speaking person.
Original: A revised International Opium Convention was signed at Geneva on February 19 , 1925 , which went into effect on September 25 , 1928 , and was registered in League of Nations Treaty Series on the same day .
Simplified: A modified International Opium Convention was signed on February 19 , 1925 , which started on September 25 , 1928 .