Veja esta postagem .
Reading through the comments of the blog you show, I saw the most elegant solution. Just need to use VirtualBox itself: Excerpted from VirtualBox User Manual version 2.14, page 108:
VBoxManage clonehd <uuid>|<filename> <outputfile> { -format VDI|VMDK|VHD|RAW|<other> } { -remember }
format: Allow to choose a file format for the output file different from the file format of the input file.
Remember: Keep the destination image registered after it was successfully written.