Changing Dynamic to Fixed VDisk?

I have installed two instances of Ubuntu on VBox using a dynamic VDD setting. Now, I find out through research and advice here, that I may get better performance by allocating a fixed amount of disk space, say 12gigs to each VDisk. Is there a way to change this setting without mirroring the drive, which I don't really follow, or deleting and re-installing the whole VM?

Thanks.

AS