from 2.1.3 to 2.5.1 any advice?

My friend is running a blog with version 2.1.3, his blogroll got hacked and asked me to do an upgrade for him. So I did but my guess is that too much has changed between versions and the end result was a big mess, categories didn’t work, the theme was messed up and so on. I reverted to 2.1.3 and I was wondering if it would be a better choice to go step by step from one release to another, however I have no idea what version in between should I prefer that includes important changes.
What about something like
2.1.3–>2.2–>2.3–>2.5.1
???

thanks