Do you like worms? 'Cause I got a feeling you just opened a big ol' can of 'em!
The objectivists are going to come and say that there's no difference, and then the subjectivists are going to counter that they can hear a difference and so there must be a difference. And then there will be ad hominem attacks, yada yada yada, and the long and short is that you probably won't be any closer to an answer.
The truth is that there isn't any difference so long as the DAC is designed to be audibly transparent. If a DAC is designed to have some sort of coloration, then there could be an audible difference. I'm going to guess, though, that most DACs are designed to be transparent.
To clarify, "audibly transparent" means that there's no coloration, that all distortion, noise, jitter, etc., are below the limits of human hearing. By definition, any two audibly transparent devices will sound identical. (Be careful when reading Head-Fi because not everybody uses this definition of "transparent.")
By the way, the only reason I'd consider upgrading a DAC is if I need (uh, um, want) different features.