When in the board program, left click on the ULP button. There are two ulp's for changing layers:
1) cmd-change-swap-layer.ulp
2) copy-layer-to-any-layer.ulp
I don't think either will work. The Help section for the second method implies you may be able to go from copper to geometry, but that won't help you. I did not play with either ulp very long. You might also check out the Eagle (cadsoft) site for other ulp's.
As ronsimpson has suggested, changing a non-copper layer to a copper layer in Eagle may require going to another program.
John