You will need access to a DETech tool to perform this action. These can be found on Jay Appell's EZPE project page. However, you will need the code of the day to authorize use of the tool which I cannot provide on a public forum. The decryption process will take 4 or more hours depending on drive size, speed, and which DETech disk you choose to use and can take over 24 hours in some cases.
If I were in your shoes I would copy the user profile and reimage the new drive rather than attempting to keep the same Windows install. In the end, this will be faster and easier than attempting to learn how to use the DETech disk, decrypting the system, and fighting any Windows issues caused by switching to a different disk.
Thank you for the reply. It sounds like we may have our route forward. I think the re-image is the way to go looking at the alternative!