I was trying to change the layer of ~30 tracks at the same time because I didn’t want to redraw them. I saw this post but there was no solution.
I figured that the data might be text based so I pasted it into notepad and found that you can find (ctrl+f) and replace all of the layers that you want. However make sure that you don’t replace anything in the list of layers in the first section.
I don’t have V6 anymore, but I think this also worked in V6. The trick is that only track segments should be in the selection, and nothing else. When only track segments are selected and you press e to edit their properties, you get the Track & Via Properties window, and you can select a new layer.