People aren't as dumb as many administrators make them out to be; they'll easily adapt to a different system, especially if they're not as technically inclined as others. Its usually the opposite that tends to be the issue (As shown on this forum); administrators and technically inclined users are the ones against changing, as they're already use to one system, and are unwilling to learn a new one that acts differently.
XenForo trains the user while they browse. Certain things dropdown as a hover, other things create an overlay when clicked, and some things work much differently then they're use to, but in the end they'll grow use to it, and probably be more fond of the new system then they were of the old.