WordPress MU

Hello,
I need to upgrade my wpmu from 1.2.3 to the latest version.

I read this page http://codex.wordpress.org/Upgrading_WPMU but I've got some question:
1. The wpmu database structure has been modified: every wp_X_posts table got an additional field. Is it a problem?
2. Do I have to redirect visitors somewhere else while upgrading?
3. Database is very large (more than 1GB). Will the upgrade script be able to modify all the tables without stopping? I can edit PHP configuration (execution time and memory limit), but I'm wondering if there are other settings to adjust to let that script run for a long time..

Thank you

Francesco

Once you've installed the new code there is an upgrade blogs option which will make all the required changes.

Did you mean 1GB or 1TB?

Thank you for the answer.
I mean 1GB.

Then there will be no problems if if the DB is only that size