Users can change their display name, but public user id is immutable.
Support tell users to open a new account, or refuse by any means to change account name.
Reddit support states :
Is it possible to change my username?
« No. It is not possible to change your username after you create it. This includes modifying your current username in any way. Please note that deactivating your account does not recycle the username. You are more than welcome to create a new account though, you can keep them both if you wish and use the same email account for verification. »
Last update on June 17, 2020