Replacing a .NET dll - vb.net

Question

I have a dll which is installed with the initial installation of my app (via an msi file). The dll contains a user key and this is 'demo' for the initial installation. When a user buys a licence he is provided with another dll which contains his name. The second dll is simply the first, rebuilt with a different name so it is the same GUID and file name.
This works fine on my win7 test machine, I can replace the dll in my apps dir and it runs correctly. I have recently provided a user dll to a new client but the replace method doesn't seem to work. He is quite IT literate so I think he is following the emailed instruction (replace the userdata.dll in your app directory with the attached) it does not seem to change the dll. He is using Win8(pro).
I had thought of sending him an Inno setup to copy the user dll into the app dir, Flags:ignorereversion regserver sharedfile
Can anyone suggest a solution or an explanation?
Later...
I have now sent him an Inno setup for the updated dll and this works. If I used the second dll method (a good idea) I would still need to have the user install it.

Answer 1

Rather than replacing the original .dll, why not provide a second .dll with the customer's specific info? The 2nd .dll will unlock features in the original .dll.
For instance, in your original .dll you might check for Customer.dll:
if(TryLoadAssembly("Customer.dll", out assembly)) {
if(Validate(assembly)){
IsUnlocked = true;
}
}
Further recommendations (and untested samples) - have Customer.dll contain a single object implementing an interface:
class Customer : IToken {
GUID Guid {get;}
// other fields
}
To validate:
bool Validate(Assembly assembly){
Type type = assembly.GetType("Customer");
IToken customerToken = (IToken)Activator.CreateInstance(type);
// check some properties
return customerToken.Guid == application.Guid;
}

Answer 2

You say it doesn't appear to be replacing the DLL. Is it UAC redirecting his filecopy into local storage?
If this is the case then the easiest way to deal with it would be to either
1) supply a batch file that can do the file copy, along with instructions to launch the batch file by right clicking on it anc choosing "run as administrator".
2) supply an executable that can do the file copy. You can either include instructions to run the exe as an administrator like the batch file, or you can include a manifest with the application to instruct windows that the file needs to execute as an administrator.
A last option, which might be worth while for troubleshooting would be to get the user to turn off UAC and try the file copy again. If that works then this user will be happy and you know what the problem is and can find an elegant solution for future customers.
I've just looked on my Win 8 laptop and the option for UAC is in Control Panel - User Accounts - User Accounts - Change user account control settings. This will give a slider which can be slid all the way to the bottom to turn off UAC.
(User Accounts really is listed twice.)