As far as I know, the web apps developed for Servlet 2.3 are still compatible with the Servlet 2.4 Container... One prominent warning will be at the servlets, which implement SingleThreadModel interface... I would suggest you to have a look at the portion that contains changes between two specs in the Servlet 2.4 Specification.... So that u will know which servlets in your web app are needed to modify...
Co-author of SCMAD Exam Guide, Author of JMADPlus
SCJP1.2, CCNA, SCWCD1.4, SCBCD1.3, SCMAD1.0, SCJA1.0, SCJP6.0