《新唐书·列传·卷五十一》
惠妃秘密派宦奴牛贵儿去对张九龄说 ：有废就有兴。公若肯援助，宰相之位可以长处了。 
妃密遣宦奴牛贵儿告之曰： 废必有兴，公为援，宰相可长处。 
Hui Fei secretly sent the eunuch Niu Guier to tell Zhang Jiuling: "To depose one, you must promote another. If you are willing to help, you can hold the position of Prime Minister for a long time."
<s>[INST] Kindly elucidate the central themes in this passage in English: 妃密遣宦奴牛贵儿告之曰： 废必有兴，公为援，宰相可长处。  [/INST] Hui Fei secretly sent the eunuch Niu Guier to tell Zhang Jiuling: "To depose one, you must promote another. If you are willing to help, you can hold the position of Prime Minister for a long time."</s>
Kindly elucidate the central themes in this passage in English: 妃密遣宦奴牛贵儿告之曰： 废必有兴，公为援，宰相可长处。
user
Hui Fei secretly sent the eunuch Niu Guier to tell Zhang Jiuling: "To depose one, you must promote another. If you are willing to help, you can hold the position of Prime Minister for a long time."
assistant