I read on the Whonix website that in order to update from Whonix 5.6 to Whonix 7, I had to download the image again. While I have no problem with that, how can I update my virtual machine while preserving the files I currently have? Is it as simple as downloading the image then setting the virtual hard drive to the old drive file, or is there a way to transfer the files securely between the virtual machines?
I get adrelanos' point that this is a Linux question, and not Tor specific. However, given that it's probably a common question for Whonix users who don't know Linux, I believe that it deserves an answer here.
First, if the old Whonix workstation has been compromised, transferring data may compromise the new one. If you're just transferring text, gpg keys and such, it's safest to transfer them via your favorite pastebin. Just archive, encrypt and ASCII armor, and then paste the text.
If you need to transfer more than that, Jason's second answer is easiest, and probably safer than mounting a physical USB drive. In the VirtualBox Storage tab for the new Whonix workstation VM, add a second SATA disk, and select the virtual HDD (VMDK file) of the old Whonix workstation VM. The old Whonix workstation VM should not be running while you're doing this.
Now start the new Whonix workstation VM. Once it boots, open a terminal and run the following:
sudo su mkdir /media/oldwhonix mount /dev/sdb1 /media/oldwhonix cd /media/oldwhonix ls
Copy whatever you want from the old filesystem to the new one. Do not copy anything that's executable, and don't execute anything from the old filesystem.
Then run the following:
cd /home/user umount /media/oldwhonix exit exit
The next time you stop the new Whonix workstation VM, remove the old Whonix workstation's virtual HDD from its SATA controller.