Upgrading from Tomcat 5.0.28

I am looking for some information on upgrading Tomcat 5.0.28 to either 5.5 or 6.0. I need to upgrade a Portal Application that is running on Tomcat but the portal application upgrade does not run on my current version of Tomcat. I have been scouring the web but can not find any info about this. I am looking to get info on the necessary steps for upgrading as well as find out if upgrading Tomcat will effect my current set up. For example, I am running Tomcat as a service and using the ISAPI Redirect so it will use IIS as the web server. Any assistance would be appreciated.