Currently, if you want to replace a widget that is already in a UI with another one in Qt Designer, your only option is to "Morph into" a similar type (ex, morph a QSpinBox into a QDoubleSpinBox), or "Promote to" a custom type.  It would be nice to be able to drag-and-drop a completely different widget onto an existing widget to replace it with anything you want.  Perhaps this requires holding down the Ctrl or another modifier key to enable this behavior.

Deleting a widget and adding a new one in the same place can be complicated when layouts are in place since everything moves around once the original widget is deleted.