http://brainwreckedtech.wordpress.com/2012/01/08/howto-convert-vdis-between-fixed-sized-and-dynamic-in-virtualbox/ http://www.webdesignblog.asia/software/linux-software/resize-virtualbox-disk-image-manipulate-vdi/#comment-474
While there is no way to actually switch a VDI between fixed-size and dynamic, you can clone the existing VDI into a new one with different settings with VBoxManage.
VBoxManage clonehd [old-VDI] [new-VDI] --variant Standard
VBoxManage clonehd [old-VDI] [new-VDI] --variant Fixed
If you want to expand the capacity of a VDI, you can do so with