I think you are looking for the reimplementation of this method:
Sets the data for the item at the given index in the model to the contents of the given editor.
The base implementation does nothing. If you want custom editing you will need to reimplement this function.
This would allow you to emit a signal here where a cell changes, both with the previous and current values.
There does not seem to be a convenience signal off-hand in QTableWidget
for this.