OK so you want or need a new user id to be associated with a system user, in this case tomcat.
These are the steps:
First mod the user tomcat to the new uid(see below):
/usr/sbin/usermod -u 755 tomcat
This has the unintended affect of removing tomcat from owing its own files so you have to change that. File permissions are done by uid not name.
So now you have to change all tomcat's files to the new UID. Remember that ufs and ext* file systems store all ownerships with the UID, not the userid string.