Prebuilt Kali Linux VirtualBox Image - Customize Virtual Machine Settings?

OK, so I downloaded the Prebuilt Kali Linux VirtualBox Image "Kali Linux 64 bit VBox" from Offensive Security. Over the past 6 months I've spent blood, sweat and tears creating my ideal Kali Linux 2.0 environment based on a cascading set of a dozen VirtualBox snapshots. Now, to my horror, I find that I need access to 4 CPU cores, but the VirtualBox Settings in this Prebuilt Image configured "System... Processors" to only 1 CPU, and it's greyed out so I cannot change it!

I don't recall whether # of CPU's was a manually-configurable setting I could have changed during importing the image, or if it was greyed out. If it IS configurable ONLY during image import into VirtualBox, the last thing I want to do is start from scratch and lose all the accumulated changes I made to my Kali environment. Any good advice on this would be extremely welcome!

Make sure you have V-TX enabled.