You do not have to save it. Just use the "clone", and click Login.
Yes, I see this functionality, but anyway it was much more comfortable for me to login to a saved session with another username (i got > 10 different usernames with the same password) without saving it.
I have a problem with new login dialog (Version 5.7.3, build 5438):
I have a bunch of saved remote servers which are being accessed via SFTP protocol using one username.
Sometimes I need to access saved host with another username once - and new login dialog doesn't allow to do this without saving changes, but it was possible in some old version (unfortunately I don't know exact version, it was about one year old)
Steps to reproduce: start WinSCP -> choose saved server -> press edit -> Login button becomes disabled until session changes were saved.
As far as I remember - earlier version just didn't have Edit button and it was possible to change credentials and log in without saving changes.
I would appreciate if you could return this old behavior.