upgrade from 3.0

Ok- this is issue is an oldie but hopefully a goody in terms of answers.

My company has inherited a WP site that is stuck in 3.0 because the dollars were never there to update the theme. From what I understand there was a switch in hierarchy between pages and categories between 3.0 and 3.1.

I haven’t worked with WordPress that long so I didn’t see this issue when it came along the first time. So, I am asking you (the WP community)- what do I need to do to successfully update the theme of the site? At the very least a link to some documentation that dealt with this issue at the time of 3.1 release.

Thanks in advance for any help.