Changing users
There can be many different reasons for changing users, and there are many different things that we can change.
If a password was leaked, then the first thing you want to do is lock the account and/or change the password of the account. If you have an application account that you suspect is no longer being used, it might be smart to first lock the account before dropping it later. This allows you to simply unlock the account if it turns out that something was still relying on this account. Locking an account is also a good way to protect a shared database against a single user who is overloading the system, for example, by writing too much data or running too many heavy queries. Then, you can lock the account, ensure the application abusing the database gets fixed, and unlock the account again.
Another thing you will often need to do is periodically change passwords. For applications, you might want to create a new user with a new password but with the same permissions...
