When we are talking of change, there are two directories: an old and a new one.
Previously Plesk pointed the domain in standard directory: /httpdocs (which is exactly /var/www/domainname.com/httpdocs). - this is "old" directory.
I have created "new" directory /site1 (or any other name) (which is exactly /var/www/domainname.com/site1) and changed the domain settings, to point to the site1 direcotry, but webite's content is still served from httpdocs directory.
What happened?