I've just created a VM and attached an existing .vhdx to it. It totally works. It even let me snapshot it. Is this a supported feature? I didn't see anything in the product documentation...
Interesting news.
I was aware that older versions could handle the older vhd-format.
Be careful though - not all functions like shrink, expand ... may be working.
I'll experiment with it tonight, perhaps. When adding the existing virtual disk, the drop-down selection list in the file chooser dialog did not have .vhd or .vhdx. But I selected "all files" then selected the .vhdx and sure enough, it worked. Interestingly I got a pop-up asking if I want to convert the virtual disk to a newer format. I wonder if it would've converted the disk to .vmdk.... I'll test that as well.