I am just starting worm farming but you need to get a good mix of Carbon and Nitrogen rich materials to get the worms eating optimally and to attract them naturally. Most windrow type farms add material to the top and then when they are ready to harvest the compost they remove the top couple of feet because that is what holds most of the worms. I think you can get them to migrate horizontally but you need to offer them something that attracts them that way. I would suggest working in some carbon based material (cardboard) and add some food variety to the new spot (garden scraps) to move them that way.
Like I said I'm a rookie but this what I have read, watched and talked about with other worm farmers.
