To clarify, Ben referred to a very specific situation which is one of the
few situations in which overwriting a PST file might work: the file that is
overwritten is blank and had not yet completed its connection to the Outlook
profile.
Unfortunately, that situation rarely exists, so most of the time overwriting
a PST file will corrupt the Outlook profile. Also, the latest version of
Outlook seems not to tolerate that procedure. Older versions did.
The only supported and recommended method to change to a backup PST file is
to open the file in the Outlook profile (either directly or via Control
Panel > Mail), set it as the default folder, restart Outlook, then close the
other PST file.