Complex sentence: In 1921 she was sold to the Canadian Pacific Railway Company, who then renamed her the RMS "Empress of China III" (or alternately known as "Empress of China II"), and began modifications, including conversion to oil firing instead of coal.
Split into simpler sentences: In 1921 she was sold to the Canadian Pacific Railway Company, who then renamed her the RMS "Empress of China". The "Empress" was enhanced with a number of modifications, including conversion to oil firing instead of coal.