Upgrading Drupal

hi all!

i administer a project-server with 500+ Drupal-installations on it.
All these installations have one, collectively used "core installation".

As the websites count was about 100 the updates took a lot of time, i.e. one whole day, and already
that was too much. But now, as there are > 500 sites, it's simply unmanageable...

Hi,

I'm trying to update drupal 6.12 to 6.13. I tried everything, from instructions in upgrade.txt, then instructions in handbook, but nothing works. I hoped this one will work, but also no luck - http://drupal.org/node/390448

Briefly, I did this in the last attempt (which followed instructions in http://drupal.org/node/390448):

Hi,

Thank you very much for helping me learn Drupal.

Hi...
We have a test site that was running Drupal 6.2; then we received notice that our database host url had changed. When I went in to update the settings files to point to this new database URL, the site wanted me to install drupal.

So I did, And I got a slew of errors. Most of them I have been able to find the answers to manually on this site (the missing columns in the cahce and url_alias tables, for example). Then there was one problem left that I couldn't find an answer for:

Notice: unserialize() [function.unserialize]: Error at offset 7 of 8 bytes in /var/www/test/drupal6/drupal-6.2/includes/bootstrap.inc on line 458

The code on line 458 of bootstrap.inc reads as follows:

$variables[$variable->name] = unserialize($variable->value);

I'm not sure if you'll need the lines around that or not.

Fatal error: Call to undefined function block_list() in /home/topnet/public_html/cinselfanteziler.com/includes/theme.inc on line 1580

this is what I get after upgrading to 6.13 from 6.12.

Nothing works now. Any advice ?

I'm a greenhorn just beginning with Drupal by reading a book. Now I'm told by Drupal 6.12 to change to Drupal 6.13. CCK and FCK modules I've already installed. How can I do that without deleting my old 6.12 Installation?
Best regards
Luca