If the center 6" of the bread board ends are glue to the table top. All you need to do is drill out the dowels from either the top or the bottom in the areas of the bread board that is not glued. Once you drill through the tenon of the table stop drilling, leaving the remaining portion of the dowel in place. Then cut shorter dowels and put them in the 3/8" holes so that they do not touch the tongue portion. There is no need to elongate the holes in the tenon. The center portion that is glued will hold the bread board ends on the table and the rest of the table can shrink or expand as it wants to. It will still look like the dowels are doing their function, but they aren't needed.