Simplify the following text for a Grade 9 native English-speaking person.
Original: When he became the senior cardinal Dean of the Sacred College of Cardinals on 26 September 1803 , he transferred to the See of Ostia and Velletri .
Simplified: Henry returned to Frascati in 1803 . In September of that year he became the Dean of the College of Cardinals and hence Cardinal Bishop of Ostia and Velletri , though he still lived in the episcopal palace at Frascati .