It would be very convenient if we could just right-click on a calculated field, such as "Period" in the screenshot below, and be able to modify the existing expression (within a context window)

Instead, I now have to do the following:

1. Click on the Data Source

2. Click on the ellipses button within the "Calculated Fields" property of the Data Source's properties

3. Find the desired expression within the "Edit Calculated Fields" window

4. Click on the expression's drop-down and select <Expression> 

That's a lot of clicks for something that is done rather frequently!