I apologize that this has already been answered, but in regards to applications already developed in Struts 1.x, will they function the same if switched to Struts 2?
There's no reason they can't exist side-by-side in the same application. There is also a Struts 1 plugin for Struts 2 that allows the use of S1 action classes, but it doesn't let us use S1 tags, so I'm not sure it's worth it.