I went form 3 to 5, and only had problems with background colors changing, and controls moving a bit,(I think the default font size changed, some custom classes didn't look quite the same, 3d controls for instance, a VFP5 version came out though. You also might have some problems if you assign constant date values, but you can avoid them by setting strict to off..that's about all the problems I had, but there's surely some others, all of them should be rellatively easy to fix.