Ok, got a little bit further this time. Creating a new VM image caused problems (couldn't get past busybox), but when I tried "qemu-system-x86_64 -hda exosvm.vmdk -boot order=d -cdrom exospc-6.1.2.14.xos" the boot process proceeded normally, but I have no idea how to upgrade the 15.3 VM image.
Could you tell me the exact steps needed to upgrade it, since install and upgrade both result in "invalid filename". Do I need to upgrade it incrementally (like 15.7, 16.1.1, then 16.1.2) or am I doing it completely wrong?