The way to do that in TCad is to move stuff from one layer to another. Decide which layer (or layers) you're going to abandon. In the "Layer" panel of the Design Director palette, highlight the name(s) of that/those layer(s), right-click and choose "Select by." Then click in the "Active" column (second from left) of the layer to which you want to move the selected object(s).
To change the layer of a Group, you should first explode it, then move its components to the new layer, and then re-Group them.