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.
In v7 there’s the Property editor panel where you can work on a selection of items and do exactly what you want. If the panel isn’t visible, open it from the View menu.
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.