I do see the prompt when booting mine. Could you have accidentally said 'no' when it popped up?
Nope, it never appeared. I tried it with two Win 7 VM's and a Mavericks VM. It didn't appear at all so I had to manually do it in the Compatibility settings pane. My first try with the Tech Preview was with Yosemite and I upgraded that manually before first boot. Could that have set some kind of pattern? I was surprised that installing the updated Tools, both automatically in Windows and manually in Mac OS didn't generate some kind of warning or advisory to update the HW version too, for compatibility.
BTW, the Tech Preview didn't help with additional screen size options in Mavericks either, which I kind of expected it would. The video drivers I recommended in another thread on Yosemite as a VM in Fusion did work in both Mavericks and Yosemite.
Update: As part of testing the Tech Preview in Yosemite (which didn't work), when I tried to open a pre-existing VM from within Fusion, I did get the Upgrade prompt before the VM attempted to run. Approving the upgrade caused the Hardware version to change in Preferences but the VM didn't actually run.
Message was edited by: MacsRule: Result of additional tests added.