I have an honest, if simplistic question, why bother explaining about Open Source? Use it, give them the number to call if they have issues, describe the new system simply as the new system, what it does and how they will be using it. Why introduce more information and potential confusion than most of them need or want? The users just want a system that's easy to use and does what it's supposed to do. The few users who are interested can be given more info if they want it, but keep it simple. The very, very few who are really interested will doubtlessly know something about, "The Google and The Intertubes".