I first thought there was an option on the selection in an input box, but I am apparently thinking of the options on the standard TextBox control on user forms. (Oddly enough, I seem to think that when I run a certain program I have that recieves a boatload of inputs, the default behavior of InputBox is opposite of yours, so maybe there is some deeply buried windows setting that I have different?)
So you could either create your own simple input form as has been suggested, and set the SelectedText property of the TextBox equal to the Text property, or you could possibly just change up the logic a bit. and do something like this:
Dim returnVal As String = InputBox("Enter new value: [" & attRef.TextString & "]" , "Edit block attribute", "")
As you can see, that would show the user the original value as part of the prompt, and the box would be empty. Then you could insert logic in the code to interpret an empty response as a No Change response, keeping the original value.
Of course that would cause an issue if the user tried to clear the previous value by leaving the box empty.
Dave O.