SBS 2011 Standard: Exchange Database Size Limit

DO NOT INSTALL HYPER V ON TOP OF SBS! YOU WILL BREAK THINGS!

With that said, you may use Hyper V in conjunction with SBS, but you must install Windows Server Standard on the bare metal and only enable the Hyper V role on it. Once this is done, create a virtual machine for SBS and another virtual machine for Windows Server standard. That second virtual machine is where you should install TMG (and only on that second machine. Installing on SBS would be a no no as it only supports 1 NIC).

You would need the following licenses (plus CALs) in order to be legit with this:

1. Microsoft Windows Small Business Server 2011 (either Standard or Essentials depending on your needs)
2. Microsoft Windows Small Business Server 2011 Premium Add-On (which despite popular misconception is an add-on to either Standard or Essentials and not its own product)
3. Microsoft Threat Management Gateway 2010

Well, I would absolutely not attempt to migrate an SBS 2011 to Server 2008, even if it wasn't a Domain Controller. So, don't try that.

But, what your customer wants can be done. Just don't try to do it directly.

Basically, you want to add a second Domain Controller to the domain, and then get rid of the SBS server as a Domain Controller.

(Before following the below instructions, it's a good idea to make sure you have a current backup of the Domain Controller. Always a good idea, just saying it explicitly.)

1. Check the Forest and Domain Functional Levels. By default, SBS 2011 creates and uses a 2003 Domain and Forest Functional Level. There's no 2011 Functional Level, so you're not actually migrating/downgrading the domain. I'd link how to do this, but I have no freaking idea how to check your Functional Levels on an SBS server.
2. Get a second server and install a version of Windows that is at least the same as the lowest Functional Level. If you plan on installing Server 2008 R2, you can even skip step 1.
3. Join it to your domain.
4. Use DCPromo to make it a Domain Controller, making sure it's a Global Catalog.
5. Transfer all the FSMO roles to your new Domain Controller.
6. Sixth, demote your SBS Domain Controller. (Again, I'd link to this or explain how, but I have no freaking clue.)
7. Done!!

Optional, but additional advisable steps:

1. Format the disk on the server that had SBS 2011. Install the same version of Windows you just got done installing. Follow steps 1 through 4 above to make it a Domain Controller. Now you have 2 Domain Controllers. You should never have only one Domain Controller, and always have at least 2.
2. Raise the Functional Level of your domain and forest.
3. Contact HopelessN00b and offer him a cut of what you get paid for this job. Seriously, he's a good guy to know, and would really appreciate this. :D