Upgrading 2.1.3 to 2.5

After getting some nasty spam inserted into my clients 2.1.3 wordpress installation I thought it best to recommend upgrading to the latest version.

The theme it uses isn’t that complex and doesn’t do anything crazy with the API. The coder that created it didn’t do a very good job with the CSS tho, but then I guess that’s a different matter.

When I do upgrade, are there any major changes to the API that I should be aware of, or anywhere where I can see a list of changes?

Any info would be appreciated

Cheers
Niall