Don't do it
I had had my HRS for several years in varying salinities. Happy, molting, eating so much short green algae they ignored their "treats" after a while and then I decided to try them in freshwater since it is repeated over and over on the Internet this is possible.
I spent two months getting them down to 1.0025 spec. gravity from I believe 1.008 or 1.010 and then went all fresh, keeping a container with the brackish water handy for two days in case of stress. They seemed fine. Day 3 they were fine in the morning, I had a busier day and did not look in again until night time and got to watch the last one die.
On some other forum, someone from HI mentions buying some from a LFS that were in brackish, bringing them home, tossing them in fresh, and having them die in hours.
There is so much info. repeated on this species on the Internet that seems to originate from only a few sources that is almost like an echo chamber. I really wonder if they even live to be 20 years old. Until someone slaps a microchip on a juvie and tracks it for 20 years, I am betting on closer to 5-8 years. The 20 years seems to be a twisting of words from a few people saying they have a large colony that has been going strong for twenty years. Note: The colony is still going, not every original shrimp.
So much "knowledge" is anecdotal compared to the freshwater shrimp info. you can get. So-and-so forgot a jar in his garage for years and they are still alive. Are they or did they breed a few times and a few of them are descendants, etc? I know mine were several years old as I got them as juvies and had a small colony where I could keep track of who was who. How do you do that with 90 shrimp?