add a layer by putting two point cursors at opposite ends from each other on the map and press your insert key (be sure to make the block color the original ground color first). You will lose everything on your map by 1 layer but i dont think that will be a problem will it? Im pretty new to the editor but i think this should work, give it a go and tell me how you did.

I accndetally built my map onto the water, thinking that 63 was the level ABOVE the water and 64 was the water. Is there a way I can shift the whole map up? Or will I have to start over D:?