I’m pretty sure nothing will change from the current functionality. The user key is automatically generated when a user is created. The userkey of a user won’t change when you change a username or any other information related to the user. There is no way to change the userkey after a user is created. The only thing Atlassian will be changing is how the key is generated.
It is also stated that this will only be done for new users in the blogpost. Many configurations and other settings are tied to the userkey so they can’t just change it without the possibility of breaking a lot of 3rd party apps and possible other things.