Making the drag and drop work in modified version
Briefly

To enable moving cell content onto empty squares in a JavaScript drag-and-drop interface, you'll need to modify your existing code. Currently, the implementation prevents content from being placed into an unoccupied cell. To address this, consider implementing checks that determine if the target cell is empty before allowing content to be transferred. This logic adjustment will ensure that existing content can be smoothly relocated to any empty squares without issue.
To facilitate successful content movement between squares, ensure that the drop functionality recognizes empty target cells and updates their content accordingly.
Adjust your drag-and-drop implementation to first check if the target cell is empty. If it is, allow movement; otherwise, prevent action.
Read at SitePoint Forums | Web Development & Design Community
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