Expand LUN over 2TB

Just wonder if someone could offer some assistance. We have a Windows 2003 Server box that requires an existing partition to be extended over 2TB so i belive this needs converting back to a GPT disk. I have read a few threads and from what i can see the current partition needs destroying and re-creating to allow this to happen.

I am no Windows expert and getting no help from so called experts at my work, so if anyone could offer assistance on this it would be appreciated.

I am just wondering if i create this new LUN of 4Tb in size, can i use SAN copy to copy the data from the existing LUN to this new one. Then once on the server the old partition is removed and this new one is presented will windows accept it as a 4Tb partition. Or would there be anything in the data (headers or records) that would not allow this to work?.