[CentOS] How to find differences in Centos Builds

Good Afternoon,

Long story short I recently upgraded my CentOS 4.4 from:

2.6.9-34.0.2.EL to 
2.6.9-42.0.2.EL and am having problems with dhcp, dns and ftp (very funky nat setup, don't ask).

My question is:

What is the best way of troubleshooting issues after upgrading?
Is there a list of changes between versions that I can look at somewhere? (obviously, but where?)

I've already done a diff between kernel variables(net.ipv4.*) and found some differences.  I've also taken a look at which packages yum has updated in the process.

Am I going to have to go through changelogs for each package to find out what I'm looking for?

There are no kernel panics, no obvious errors and all services appear to start fine....
Thanks in advance.

Andrew Elliott.