Hella:
In my practice, after I have modeled the existing conditions, I keep a copy of that file. In the new file for the design, I just move the parts to be demolished to a Demolition layer (changing some of their attributes), and leave the parts to be retained on their Existing layers. I can open the Existing file and copy and paste into the Design file if I need some existing parts again due to design changes. This means that the majority of the elements just stay on their original layers. To move the entire project to new layers seems unnecessary. If this does not work for you, then you should post more information about what you are trying to accomplish.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
HTH
David
