Upgrading Drupal

I'm looking to upgrade my drupal site from 5 to 6. The problem is, I can't have the site down for very long and I don't want to risk a straight upgrade (when I built my site originally I hacked together a lot of bad code and I know there'll be problems). What I want to do is basically install drupal 6 on the server where I can work on it, then when everything is in good shape, move it in place of the drupal 5 site. Can this be done and is this the best way to go about it?

I have a very weird issue. Upgraded Drupal 6.09 =>> 6.15 total stock no extra custom modules.
No errors or problems during the upgrade.
Everything looked great as I was logged in as Admin. Then I had a user call me and wonder why they could not access any pages. Each time they would get an access denied and the "Drupal Welcome Screen" was back when you logged out.

I have just upgraded from 6.8 to 6.14. Everything appeared to be fine except that I get this message when I click on forum-

Fatal error: require_once() [function.require]: Failed opening required './includes/bootstrap.inc' (include_path='.:/usr/local/php52/pear') in /home/user1/public_html/index.php on line 15

Also none of my pictures show up now.

I was searching the forum and someone said that they didn't have their settings.php file and after re-uploading it, everything worked. I do not have this file either, nor do I have a back up.

I have 3 drupal websites. They were created with different version of drupal months ago (one website owns one drupal installation). Now I want to upgrade to drupal 6.15 and integrate these 3 websites into one drupal installation (/drupal/sites/site1,/drupal/sites/site2,/drupal/sites/site3).

Can I just modify settings.php in each slave site folder to point to corresponding database and move themes/modules into new drupal then enable later?

Or anything missing?

Many thanks.

After upgrading to 6.15, I can no longer access mysite.com/admin. It shows a blank screen. Once I am logged in I can access all of the pages by using the navigation, however the /admin just shows a blank white screen. How can I fix this?

Hi, I performed the upgrade then added few pages to my site.

Afterwards, I wanted to generate and submit a new sitemap.xml but the free generator I usually use always says "An error occurred while generating sitemap".

All the other sites I run with Drupal work, except this one that I recently upgraded.

Has anybody else experienced the same?

Thank you.