I am sure you understand that .ui files are compiled into your program and are not used at runtime, right?

You can simulate fade in / fade out using the QWidget::windowOpacity() property and QPropertyAnimation. Animate the opacity for the widget you want to fade out to go from 1.0 to 0.0 and at the same time animate the opacity for the widget you want to fade in from 0.0 to 1.0. When you want to do a dissolve, set the opacity for the fade in widget to 0.0 and the fade out widget to 1.0, call QWidget::show() on both, then start the animations. You can connect the fade out widget's QWidget::hide() slot to the QAbstractAnimation::finished() signal to properly hide it once the animation ends.