This is one of a tricky part first, if you are new to QCad.
But it is simple as it should be.
Lets assume: You have things in Layer 0 but also things you want to have in let say "centerlines".
1. Unselect all "tn".
2. Select only the things you want to move to a specific layer "centerlines".
3. Menu Modify, Attributes "at".
4. Choose the layer you want to move to here "centerlines".
5. That's it.
So 1. select then "at". If you want/need to edit also properties, then you can use "pr". This is very fast.
If you are more trained in using QCad, you will define first the Layers you need, then every time you draw:
1. select the right Layer.