WinSCP logins automatically only if you start it with your session name specified on WinSCP command line. You possibly have a desktop shortcut for that? So start WinSCP from your Windows Start menu. Then you can go to the login dialog and edit your stored session.

I can't find this Advanced Site Settings dialog for an existing server. WinSCP logs in automatically for me, so I don't see the login dialog.

How do I open the Advanced Site Settings dialog? Is it on a menu somewhere?

OK, I'll take a look at those settings.
But another *HUGE* advantage of using the archive attribute is that WinSCP wouldn't have to compare time stamps on each file on both the local and server computers. On a large site like ours, (50K+ files), our current client can check all 50,000 files and upload the few changed ones in only 20 seconds.

You can tweak the time offset in session settings:
https://winscp.net/eng/docs/ui_login_environment
See also https://winscp.net/eng/docs/timestamp

Would like to be able to tell WinSCP to transfer files if and only if the archive attribute is set. Rationale: When our transfer options were set to "New and updated files only," we discovered the server files had time stamps that more than an hour ahead of the actual time.

Not sure why the server time stamp was wrong, perhaps due to Daylight Savings Time or a time zone issue. Whatever the reason, it prevented WinSCP from performing the transfer.

This is a critical feature for us. We were going to use WinSCP to update our Web site, but if a file were changed twice, just a few minutes apart, the WinSCP would not transfer the file a second time because the server copy of the file seemed to be newer.