here is the simplest and fastest method i've found.
1.) create a randomly named new "hard drive" in the storage settings for whonix workstation 7.
2.) boot up whonix workstation and format the new drive like you would in any other linux installation. mount it. copy all the work, email folders, etc. that you wish to save to the new drive. then, shutdown whonix workstation 7.
3.) open the virtual media manager in virtualbox and "copy" the new randomly named hard drive for whonix workstation 7 to a vdi file named something like "whonix8temp.vdi."
4.) attach "whonixtemp8.vdi" to yyour storage settings for whonix workstation 8.
5.) boot up whonix workstation 8, mount the new drive, and copy your work over.
done. you can erase the new hd images you made in the virtual media manager when complete.