Updating Out-of-Date WordPress Site

I am working to revamp an online publication that is based on a self hosted wordpress.org site:

HERE

This site uses an out-of-date version of wordpress and uses the Thesis 1.6 theme. We are having all kinds of issues with the back-end integrity of the site and our front end content. Attempts to update to latest version of wordpress always seem to cause glitches and conflicts with the Theme.

I am preparing to install Twenty Eleven theme, update wordpress, upgrade to custom CSS, and then instal a child theme (i’ve done some research, read some articles on how to do this, think I understand how and why) based on twenty eleven. Thereby hopefully resolving all the issues and securing some stability for the site going forward.

Here is a mock up I’ve down with content from the publication to experiment with CSS mods and make sure the sight will still look right after the process is complete.

Is there anything I should be aware of, watch out for in this process? Anything I might not have anticipated?

Is there another way to stabilize and update the site as is with the Thesis theme?

Really appreciate your help. Thanks

The blog I need help with is proofspacecalendar.wordpress.com.