Simplify the following text for a Grade 6 native English-speaking person.
Original: In 1951 and 1952 , he conducted again at the Bayreuth Festspielhaus where he changed the seating plan for the orchestra that had been compulsory since Wagner made it in 1876 .
Simplified: In 1951 and 1952 he conducted at the Bayreuth Festspielhaus .