The reason why the Cixi Society changed its title is not clear, because there is no definite basis in this regard. Speaking of Cixi, she is also a person who makes people hate her with itch, because she single-handedly held power and humiliated China at that time. However, the concubine Cixi is indeed more special than many people. Even the title Cixi, which she never changed throughout her life, was changed, but the reason is indeed a mystery.
Generally, the titles of concubines in the harem are determined by the emperor and will not change. When a concubine in the harem reaches the rank of concubine or above, the emperor will basically choose a title to give to the concubine and then honor the concubine accordingly. However, some women with lower status are generally called by their surnames. For example, some women with high status in "The Legend of Zhen Huan" have their surname followed by their status.
Once the title is given, only the status will change. Anyone who has watched many Qing palace dramas knows that if an emperor grants a title, the title will usually be preceded by the title followed by the concubine's position. Just like Zhen Huan's title was "Xi", so when she later became a concubine, her honorific title was still "Concubine Xi". Only after Zhen Huan became the Queen Mother did her honorific title change, so Cixi's change of title was a very special existence.
Cixi’s change of title may be due to taboo. Because under normal circumstances, concubines will not change their titles, and all decisions are actually in the hands of the emperor. Moreover, Cixi was indeed relatively favored at that time. Many concubines at that time were given the word "Li", but Cixi was very special. Perhaps it was just because Cixi's name conflicted with some concubines in the past that she was changed by Xianfeng. But these are just speculations because there is no real basis.