if old layer exists, and new layer is already there, it takes everything on old layer and puts them on new layer.Basicly this means that you want to merge layers. If layers are used in blocks or xdata then this is a lot more complicated than renaming a layer.
(or (member "laytrans.arx" (arx)) (arxload "laytrans"))
(acet-laytrans "YourFile.dws" 7)