I suppose the map is made up of smtiles on front layer (typical for those newer Chinese maps) and you can't place those on back layer (back layer accepts only big tiles).
You only need to move tiles away from the front layer and in that case you could move them to middle layer. Not sure now which version of the map editor I put this feature in but the bulk conversion is in there and in the case of smtiles only, you wouldn't even need to fix client to display them properly (need to do that for smtile image strips, tiles taller than 32 pixels).
I suppose the only reason for doing what you ask is to be able to place objects on the map and objects are all built on front layer and tags would interfere (not for all objects but trees and such like would make a mess). If I am right in guessing about smtiles and object placing, then the easiest solution would be to convert smtiles to middle layer.
Some maps that are built from smtiles all on front layer use a tricky method to still have objects on them that your char in game can go behind. If that is the case, then those objects would need to be left on front layer (or returned there after bulk conversion to middle layer - more simple that way) because currently mir client wouldn't display them correctly on middle layer. But that is a simple client fix I have figured out a while back.