Upgrading Drupal

I was using zen_classic as my theme and it was heavily customized. I upgraded to 6.13 today and when it was done I got these errors:

warning: array_map() [function.array-map]: Argument #2 should be an array in /home/itug/public_html/modules/system/system.module on line 966.
warning: array_keys() [function.array-keys]: The first argument should be an array in /home/itug/public_html/includes/theme.inc on line 1771.
warning: Invalid argument supplied for foreach() in /home/itug/public_html/includes/theme.inc on line 1771.

I have successfully upgraded a site from 5.x to 6.x, but am running into the following problem with my theme:

On the front-page node listing, the old nodes (created in drupal 5.x) do not have a node-teaser class applied to them, but all new nodes I create (in drupal 6.x) do. Is there a way I drupal can add the node-teaser class into the old nodes? I would be fine with even a manual (non-scripted) solution if there was one.

If anyone has run into this before, any tips would be appreciated. Thanks.

Im running into browser usability problems.

I am using the AboutPeople theme with Drupal 6. When i view my website on different computers (with Firefox or Internet Explorer), my website looks different. Everything lines up perfectly on my computer. But, on several other computers, the secondary-links rollover to the next line, and text appears too big or too small etc.

1. I am using medium-size fonts in Internet Explorer. But, i noticed these other computers are using small, extra small, large or extra large fonts.

The following is from upgrade.txt and is also confirmed in the onsite docs.
5. Disable all custom and contributed modules.

6. Remove all old files and directories from the Drupal installation directory.

7. Unpack the new files and directories into the Drupal installation directory.

Just to go from 6.10 to 6.13 I have to do all this. Maybe I am misunderstanding, but "Remove all old files and directories from the Drupal installation directory." to me means remove my entire Drupal installation? Yikes?!!!

I upgraded from Drupal 6.12 to 6.13. Everything seems to be working correctly except for one thing. When I go to Administer->Site building->Modules, instead of getting the list of installed modules and their enabled/disabled status, I get the following error message:

Fatal error: Only variables can be passed by reference in /var/www/.../sites/all/modules/admin_menu/admin_menu.inc on line 536

I have made a lot of changes to theme files, mostly *.tpl.php files in my Drupal 5 installation, and I want to upgrade the site to Drupal 6.

Can I expect to transfer the changes easily and quickly?