The grain becomes slightly finer, the maximum density decreases and the contrast increases slightly.
With one-shot development, this isn’t particularly significant.
Generally speaking, shorter development times result in a flatter characteristic curve for the negative, whilst longer times produce a steeper one.
This principle must then be weighed up against the properties of the developer and the film.
D76 is relatively soft in itself, whereas Neopan is a hard film.
With this dilution, you should get a usable result, but not the optimum performance the film is capable of.
I think the overall density of the film will tend to be rather low.
However, I haven’t tried the Neopan and D76 combination yet, so I’d be very interested to hear your results.
But as always, darkroom work is 75% faith, and everyone has their own preferences.
D-76 data sheet in English
Personally, I would use Microdol as a fine-grain developer for the Neopan.
