When and why did Peking turn into Beijing?

The written name of the city in the Chinese language never changed. However, the Mandarin pronunciation for the name changed over time and was eventually replaced in Western translations.

According to Wikipedia (which disproved what I personally thought), it originally was Peking, then Beijing due to a linguistic morph. However, in 1928, it was renamed Peiping to remove the "jing" or "king" word which means "capital". In 1949, the Communist Party of China renamed the city _back_ to Beijing/Peking.

Bottom line though, is that there are still many authoritative sources that list the city as "Peking", even though linguistics have changed and it's pronounced "Beijing" now.

The name Peking was an Anglicized version of the Chinese name Beijing - it never was Peking because Westerners got the name wrong and it proliferated for centuries