There are two ways to do this - the first, as you mentioned, is to engrave in the opposite direction by changing the Scan Angle in the image settings to 180 degrees. That will run top down instead of bottom up.
The more recent way to do this is to use the preview - drag the slider to where the job stopped, then click the ‘Start here’ button at the bottom of the preview window and it will run the job from that point forward.