In linux you can change disk formats whatever you want with tool called qemu-img.You can install it with your package manager
$ apt-get install qemu-img
$ yum install qemu-img
$ qemu-img convert -O qcow2 my_source_image.img my_destination_image.img
If you privilege for images you can use sudo with qemu-img
it is very easy -O option says that change to qcow2 if you want change it to vmdk or qed.
Tip:If you created disk image int raw format disk file is full size which is very bad if you downgrade(decrease) disk size to used size you can usage command above.