sclee reported that the Copy as new furniture plug-in wasn't able to copy rooms anymore. Instead of releasing an update of Sweet Home 3D, I modified the plug-in and enabled it to work with Sweet Home 3D 4.6 and previous versions.
Please download it from here :
http://www.sweethome3d.com/plugins/Copy ... n-1.2.sh3p
Sorry for any inconvenience this issue may have caused.