Just a note in case anyone else runs into this.

I upgraded AiryBlue from a patched 1.0 install to the latest 1.7. But it changed my 3-column layout into a single-column layout!

My first thought was that something strange was happening in the CSS, but it was much simpler. Making the upgrade changed the names of the sidebar regions. So 'left' becomes 'first sidebar', and 'right' becomes 'second sidebar'.

You'll likely see a list of error messages about "The block was assigned to the invalid region right and has been disabled."
Make a note of them, go into the Blocks admin screen, assign the blocks listed in the error messages to 'first sidebar' or 'second sidebar', and everything is fine again.