For simple migration, it's not typically a major task. I cannot even remember what changes I made several months back when I did this to one of my major apps. The app itself runs unchanged on both Tomcat6 and Tomcat.
Obviously, if you want to start exploiting features that are new to Tomcat7 it can get more complicated, but a straight port shouldn't be too traumatic.
When it comes to destroying a civilization, gas chambers cannot hold a candle to echo chambers.