Well it’ll probably be useful to set your LogLevels to debug mode and view the apache log file after the upgrade:

http://httpd.apache.org/docs/2.4/mod/core.html#loglevel

Also look over these websites:

IBM HTTP Server Upgrading to Apache 2.4

Apache Upgrading to 2.4 from 2.2

There’s a lot of exciting new modules and changes to existing modules to check out in Apache 2.4

New features

Gotchas

Also look out for issues on Include directive thanks to @ChukShirley .  You might have to change those to IncludeOptional.