tiě​dǎ​de​yá​men​liú​shuǐ​de​guān

officials come and go

铁打的衙门,流水的官 鐵打的衙門,流水的官 tiě​dǎ​de​yá​men​liú​shuǐ​de​guān
officials come and go
Literally "the government office is made of iron, while the officials flow through like water"; used to describe how institutions remain stable while their staff and leadership are constantly changing.