Upgrading Drupal

I am not sure if this belongs in the Upgrade forum or the Install forum but here goes...

Can I have the following on a single server / IP address?
* current website: mysite.org --- v5.x
* future website: newdesign.mysite.org --- v6.x

I've been searching the forums and maybe I am just missing it but so far I have not found an answer. I have found a lot of discussions about multisite installations sharing a common codebase but I am looking for how to create a multisite with different versions of the Drupal core as follows:

I am the volunteer webmaster for a non-profit site that someone else built using v5.x and I have been given permission by our Board to completely redesign and rebuild it (the old design does not reflect many significant changes in the organization). What I would like to do is quietly build and test a new v6.x site from scratch but have it on the same server under a separate subdomain (ie: newdesign.mysite.org).

My priorities are to (a) not disrupt the current site, (b) not worry about porting any data as I plan to seriously redesign the forms anyways, and (c) transistion to the new website (when ready) overnight. Can this be done and if so how?

BTW: If it makes any difference our server host uses CPanel for administration.

I have been trying to upgrade 2 sites to mysql 5 so they can then switch to 6.8 and I am having issues. A friend upgraded the db to mysql5 and it works fine with Drupal 5.12 but when I switch the DNS to the new folder it get a bunch of errors. The new folder already has 4 sites running on it just fine.

I am using a backup DB on a dev.mysite.com running in the 5.12 directory. I disabled modules and switched to a default theme. But when the DNS flips to the 6.8 directory I get the errors below and I cannot update.php (goes to a blank page /update.php?op=info) so what do you suggest? Did I miss something?

What is the proper process when upgrading the DB and Drupal?
Here are my errors:

Warning: Unknown column 'serialized' in 'field list' query: SELECT data, created, headers, expire, serialized FROM cache WHERE cid = 'variables' in /homepages/xx/dxxx/xxx/drupal-6.8/includes/database.mysql.inc on line 128

I am fairly new to Drupal however it is clear to me that Drupal is a superior CMS.

The issue I have is that I can not find the information I require to clarify and resolve the following:

1. When I upgrade my database (from lets say) test to production, I can not import the database without first dropping the existing database which means that even if I set my site to offline mode during the maintenance period I lose my sites offline page due to the fact that the nodes are removed by dropping the tables/database.

I tried upgrading a copy of site on my local pc from 5.7 to 6.8

I got these errors on running update.php

* user warning: Column 'vid' cannot be null query: UPDATE drupal_term_node t SET vid = (SELECT vid FROM drupal_node n WHERE t.nid = n.nid) in C:\wamp\www\yeawo\modules\system\system.install on line 1150.
* user warning: Duplicate entry '5433-2182-' for key 'PRIMARY' query: ALTER TABLE drupal_poll_votes ADD PRIMARY KEY (nid, uid, hostname) in C:\wamp\www\yeawo\includes\database.mysql-common.inc on line 374.
* user warning: Duplicate entry '1-0' for key 'PRIMARY' query: ALTER TABLE drupal_term_node ADD PRIMARY KEY (tid, vid) in C:\wamp\www\yeawo\includes\database.mysql-common.inc on line 374.

and further

Update #6043

* ALTER TABLE {flood} ADD INDEX allow (event, hostname, timestamp)
* ALTER TABLE {history} ADD INDEX nid (nid)
* ALTER TABLE {blocks} CHANGE theme `theme` VARCHAR(64) NOT NULL DEFAULT '', ADD UNIQUE KEY tmd (theme, module, delta), ADD INDEX list (theme, status, region, weight, module)
* ALTER TABLE {blocks_roles} ADD INDEX rid (rid)
* ALTER TABLE {filters} DROP INDEX weight
* ALTER TABLE {filters} ADD UNIQUE KEY fmd (format, module, delta)
* ALTER TABLE {filters} ADD INDEX list (format, weight, module, delta)
* ALTER TABLE {node} DROP INDEX status
* ALTER TABLE {node} DROP KEY nid_vid

I'm trying to update from 5.x to 6.x (5.14 to 6.8), and after running update.php I get the customary

Updates were attempted. If you see no failures below, you may proceed happily to the administration pages. Otherwise, you may need to update your database manually. All errors have been logged.

But since I see several failures below, I need to (less happily) update my database manually. How do I do that?

I'd like help with two things with this forum post:

• We should have a handbook page on what to do in this situation, with whatever general advice can be given. I'll write it, but right now I don't know what it should say
• And, naturally, I'd like help with my specific problem

General handbook page

Start with a few pointers on how to get at your database (for instance Cpanel or command line + http://dev.mysql.com/doc/index.html).

My first questions:

There were a lot more error messages on the landing page I got to after update.php, than in the log. I assume both sets are relevant, and that the user should copy them all to a text file before doing anything else. Correct? Or can the user assume that all she needs to know is in the log?

Would appreciate help to recover website.
Upgraded from 5.12 to 6.8
Cannot login into site.
Site displays a long list of errors. I do not understand them.
Cannot run update script. Instructions say to do
"Note: if you are unable to access update.php do the following:
- Open your settings.php with a text editor.
- There is a line that says $update_free_access = FALSE;
Change it to $update_free_access = TRUE;
- Once update.php is done, you must change the settings.php file