VHDX Support for W2008R2 Guest

Question

I'm a little confused about the support for guest o/s's using the VHDx format. I have seen quotes on this forum saying that any supported guest o/s can be run using VHDx, yet when I create a virtual HD using W2012R2 Hyper-V, it says that W2012 is the
oldest o/s to support VHDx.
Currently, I have a pre-production environment running on a Hyper-V W2012R2 platform. All the guest VM's are (Gen 1) W2008R2 servers. Currently I am using the VHDx format for the boot and data disks on these VM's. Everything seems to be running OK now, but
I am concerned that if this is not a supported configuration that I may have trouble in the future.
Can someone clarify if my environment would be supported by Microsoft going forward, and if not, can I simply convert the existing VHDx volumes into VHD's using the Hyper-V Manager GUI?
Thanks!

Answer 1

...well surprise surprise!
After creating the above forum posts, I decided to submit my question to Microsoft Support via email. It took a few email exchanges until I was relatively confident that Microsoft fully understood my question. In the end they claim that the restriction of
VHDx to W2012 or newer systems is based upon the "host" o/s, NOT the "guest". The following is an excerpt from my last email to them:
"Given the fact that my environment is not taking advantage of any “advanced features” of the VHDx format and that it appears to be
working fine with my W2008R2 guests in my testing, will Microsoft support us if we run a production environment with W2008R2 guest VM’s using the VHDx format?"
And this is their reply:
"For what you saw, I’d like to explain that it is aimed at Hyper-V host while not Guest VM. That’s to say no matter which OS version has been installed on guest VM, as long as the hyper-v
host OS is windows server 2012 or later version, you can use VHDx format and take advantage of it. What’s more, for your environment, it is absolutely supported by Microsoft."
Services
Jason Zeng, Windows Support Professional  Microsoft Email Support Email:
[email protected]
portal:support.microsoft.com/oas/
My working hours are 9:00 PM - 6:00 AM EST, from Sunday to Thursday.
Delighting our customers is our top priority. We welcome your comments and suggestions about how we can improve the support we provide to you. If you would like to give us some feedback,
please email my manager Peterson Wu at [email protected].
Thank you.
Note: E-mail support is NOT suitable for urgent or time- critical issue. If the technical problem is urgent and you feel it more efficient to discuss it via phone, you may convert this to a
phone support case by contacting us at this number: 1-800-936-4900. Thank you for your cooperation.
In the part about "for what you saw...", the Microsoft rep is refering to the "New Virtual Hard Disk Wizard" dialog box that warns "This format is not supported in operating systems earlier than Windows
2012." Although not clearly identified in this dialog box, the warning refers to the host o/s. (I really wish they would reword this warning in a service pack, etc.)
Once again, I thank everyone who has provided me with feedback on this topic. I hope this thread helps to clarify the issue for other IT professionals because previously I could not find a straight answer to this question
anywhere else on the web. 
 -tm