There are several goals why upgrading from PLESK 7.5.2 to 8.4. In my case the most important were the native PHP5 and MySQL 5 support. The migration manager in the new PLESK version does almost all the work easy and intuitive for you. But only almost all because there are several issues.

So you should have some time left to do all the fine tuning stuff you have to do. Several sites have not worked after moving to their new home.
The most trivial comes first: The standard path of the stored vhosts/sites has changed. So you need to adjust it. You probably think: Nothing easier than that! But there is a small trap: The new path is only visible (especially via console) after you have been in it for the first time. That’s why I have no better idea than to guess it. The /srv/www/vhosts directed me to the right location. This should be the default value so it save some time for some guys. The path problem rises especially in the custom vhost.conf files: Even these path haven’t been adjusted by our migration manager. So you should get into the issue solving at this place.
You may have recognized that several default settings of PHP have been changed between PHP4 and version 5. The most popular surely is the deactived register_globals. In combinatation with osCommerce the other similar problem is the needed register_long_arrays. These values can be activated by adding to the /srv/www/vhosts//conf/vhost.conf file:

<Directory /srv/www/vhosts//httpdocs>
php_admin_flag register_globals on
php_admin_flag register_long_arrays on
</Directory>

As mentioned in a post in the osCommerce forum you have to comment out the
$this = null
in admin/includes/classes/upload.php.
This brings the menu of osCommerce sites back to life.
Another thing you gets used to is the surely the deactived error displaying by Plesk: No PHP errors will be shown to the user. To invert this you have to insert another line into the Directory directive:
php_admin_value display_errors on
Note: You can make this effort also by activate user error messages or something like that in the respective domain configuration of the Plesk backend.
Probably you will get a 1054 error now. A workaround for this is given by the osCommerce Community. Newer versions should not have this problem, but I think an osCommerce update is not really an practical alternative.
But we miss still something very important: PEAR! Plesk brings it by default, but it is not included by default. The line

php_admin_value open_basedir "/srv/www/vhosts/httpdocs:/usr/share/php5/PEAR:/tmp"

in the vhost.conf brings this great framework back to you.
Finally, you must not forget to take all these changes effect: