Simplify the following text for a Grade 9 native English-speaking person.
Original: He was made Cardinal-Bishop of Frascati on 13 July 1761 , and eventually succeeded to the See of Ostia and Velletri on his appointment as Dean of the Sacred College of Cardinals on 26 September 1803 .
Simplified: He was made Cardinal Priest in 1748 , and Cardinal-Bishop of Frascati on 13 July 1761 . 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 .