Is “Demoted for requesting the establishment of the crown prince” true?

In chapter 20 of The Generals of the Yang Family, it is written that Feng Zheng was demoted to Lingnan for requesting the establishment of the crown prince: 'Feng Zheng and others submitted a memorial, begging to establish the crown prince early. The emperor was angry and demoted him to Lingnan.' However, History of Song, Biographies of Literati, Volume 3, records that it was for requesting the establishment of Prince Xu Yuanxi, and specifically demoted to Duanzhou: 'Together with Left Remonstrant Yin Huangshang, Feng Zheng, Right Remonstrant Wang Shize, and Song Hang, they knelt in the court to request the establishment of Prince Xu Yuanxi as heir apparent. Their words were wild and rash, and Emperor Taizong was angry. ... Feng Zheng was appointed to Duanzhou.' The novel omits the specific candidate and vaguely refers to the place of demotion as Lingnan.