As with many other aspects of finance, a general rule for fund managers is to treat their investors fairly.
However you may achieve this via two options assuming you take the relevant precautions:
Create a new share class that has lower fees but this should be balanced with other terms such as less advantageous liquidity conditions or a higher minimum investment.
Use a side letter - an arrangement between the fund manager and a certain investor that may stipulate a favourable fee structure by virtue of an investor's size or strategic importance to a fund.