Hi
Use-Case 1: I wanted to give someone temporarily access to my vault. I gave him my password. Now I want to revoke this access. So I change the password.
Use-Case 2: my password got leaked. I have to set up a new one.
This is correct. If you want to re-encrypt your vault, you have to set up a new one. Keep in mind: if your vault is online, this means that the complete amount of data in your vault is once downloaded (if not already local available), and re-uploaded.
If you restore your old masterkey file from a backup that was done before the password change, you can of course open your vault and see all its content with the the old password. but you cant open it with the new password any more. The key to your vault is always the masterkey file and its corresponding password. You cant open your vault only with the file, or only with the password.