Can I do this without having to reinstall? If so please tell me how.

Why do you want to change the serial number?In AutoCAD 2004 an installation requires a serial number and an authorization code which I believe is eight characters long.  Do you have these written down somewhere?

No, that's the problem. It's on behalf of a company who are refreshing their PCs. Am I right in thinking each activation code is generated from a particular serial? So if a couple of users have been given the same serial, it will need to be reinstalled on one of them?

The activation code is NOT generated from the serial number for AutoCAD 2004 and AutoCAD LT 2004.Is this a network license?

So I can just give any Autocad 2004 LT activation code...? But have already tried that and it didn't work. No, it isn't a network license.

In theory I suppose you could.  In practice?  Well I don't really know.The company should have the serial number and the authorization code on file somewhere.  Didn't they keep the original media and the box it came in?

They don't have the authorisation codes on file, no. They do have the original media and box. But I guess it looks like a case of the simplest solution will be to have to reinstall the software and re-enter the serial number again.

No authorization code?  Then it is likely your client may have to place a call to AutoDesk for some assistance.

I know, we have established that already :) my question was, will I need to reinstall to change the serial number.

Yes, you'll need to reinstall.