When updating from one major Apache Tomcat version a newer one, please make sure that the JVM that is installed on your system supports at least the required Java version. While it is possible that older versions of Tomcat may not be compatible with newer JVMs, all the currently supported Apache Tomcat versions (6.0.x and 7.0.x) are known to run correctly on Java 6 JVMs.
When migrating from one major Tomcat version to another (e.g. from Tomcat 6 to Tomcat 7), you should not blindly copy the configuration files from the old version. The recommended approach is to start with the default configuration of the new version of Apache Tomcat and to adjust it as necessary.
When migrating from one minor release to another minor release within the same major release (e.g. from Tomcat 7.0.14 to Tomcat 7.0.16) you can retain the configuration files, but you should check to see if any defaults have changed and/or if any new elements have been added and adjust your configuration files accordingly.