upgrading busy site without loosing comments

I'm asked to upgrade a large and busy WP install from WPMU 1.2.4 to WP 3.2. The site has 100 blogs, and articles are commented every minute (it's a large national media house). The challenge is not to upgrade. I did that om a test server, and it's all good.

The challenge is to upgrade the live site with minimal downtime, and without loosing comments.

Does anyone have any comments or experience to share on such a process?

In order to avoid disruption of the live site, I'm picturing it like this:

1) the authors on the live site are told not to publish new content during the upgrade
2) I copy the db and files to a local server
3) upgrade all (in multiple steps)
4) when ok, I move upgraded DB and files back to live server

But I'm not sure what to do about commenting during the process.

Ideally, I would want to let commenting go on, and them copy the new comments over to the upgraded site. But there are 100 blogs, each will receive 100-200 comment during the process. So this needs to be an automated (and reliable) process

Or I could disable commenting across the site during the process, but How do I best do that (also for existing posts, and site wide) ?

Could I set up a filter to dynamically change the comment storing, to redirect comments to a temp. database table during the upgrade, and then merge the content of that with the new original comments table afterwards (remember the two comment tables will then correspond to different WP versions, so probably they'll have different structures) .. probably not a good idea..