Did you have a look at http://httpd.apache.org/docs/2.4/upgrading.html?
When doing the same a while ago, I found that the main change for me was the access control, e.g.:
In this example, all requests are allowed.
2.2 configuration:
Order allow,deny Allow from all
2.4 configuration:
Require all granted
You might find that you have such access controls to change in your config files.