If you have used a OEM License you need to use OEM Product Keys as you can not change between the different versions of Licenses.
You can however use something like Key Finder to change the product Keys on the Different Workstations so that you comply with M$'s Licensing Agreement.
http://magicaljellybean.com/keyfinder/
Here I'm assuming that all your systems came with OEM Licenses new and you are just using the same Image for all. This is considered by M$ as Piracy even if you have COA's on every Case. It's expensive particularly if this is a business and M$ Legal Loves to hit places like this for stealing from them. M$ have their own Products like this to change Product Keys but keyfinder is much easier.
As for moving to a Volume License they have a different series of Product Keys and you can not change a OEM to a Volume License Product Key without creating a new Image and re imaging every computer in the Office. This is because Volume License Product has different Code in it's Base to OEM or Retail product. Of course you'll need to remember Office Products here as well as Windows Product as they are treated as Different Products and if caught that will be 2 Different Counts of Piracy. If there are any other M$ Products used by this place that will just add to the list as well when they come knocking on your door.
This really should have been thought about before starting this though if you just use the one image and change the Product Key to match the COA on the case there is no problem.
OH and BTW WGA will pick this up and send a report directly to M$ and their Legal Department will eventually catch up with you and Audit the company.
First Offense when caught starts at a $250,000 settlement and listing the Companies Principles and their Home Address in the M$ Enforcement Alert. But this goes on for at least 12 Months before M$ will agree to settle so the company has a large legal bill to settle as well.
Of course if it's not the first offense M$ are not so nice to deal with. ;)
Col
