Simplify the following text for a Grade 6 native English-speaking person.
Original: Though a number of lineup changes followed , the three original members remained together along with James LaBrie and Jordan Rudess until September 8 , 2010 when Portnoy left the band . In October 2010 , the band held tryouts for Portnoy 's substitute drummer .
Simplified: In September 2010 , drummer Mike Portnoy left Dream Theater .