Complex sentence: He subsequently reverted to the title of Prince following his abdication from the throne in 1955, and was bestowed the title of "Samdech Preah Upayuvareach" by his father and successor in 1955, which translates as "The Prince who has been King" in English.
Split into simpler sentences: He subsequently reverted to the title of Prince following his abdication from the throne in 1955, and was bestowed the title of "Samdech Preah Upayuvareach" by his father and successor in 1955. The title translates to as "The Prince who has been King" in English.