I would not be at all surprised to hear that a new-era D-40 sounds different (and if it were my ears, better) than a 70s example. Starting around 1970, Guilds got heavier, with thicker finishes, and their voices (again, to my ears) changed for the worse. It has less to do with the woods used than with other aspects of the build formula*, which Fender has been changing since acquiring the company, and especially after production moved to Tacoma. I've been playing Guilds since the mid-sixties, both the dozen or so I've owned and the many dozens I've played in stores or at jams, and the post-Corona samples I've encountered sound like the instruments I played before 1970. My prediction about a 70s/current D-family comparison is that the 70s voice is more compressed and perhaps a bit nasal, maybe even shading over into brassy. The current models I've played have a fatter, more balanced sound, ranging into sweet, but still with a bit of Guild nasality. Which, by the way, is what my 1965 D-40 sounds like right now.
*Most of the builders I've talked to--and when I was doing music journalism, I talked to a lot of them--have told me that wood variety is only a part of the formula that determines sound. It does matter, but the differences are often subtle, and I have been told by more than one luthier that two instruments with tops taken from the same flitch of wood can sound quite distinct. That's why voicing a top is such a tricky art, involving thickness, taper, and fiddling with the braces. A builder friend made three as-close-to-identical-as-possible examples with walnut, mahogany, and rosewood back/sides, and they did have distinct voices. But they were more like each other than the other nine instruments from that batch, which featured different body sizes and bracing patterns. I played all of them and bought the rosewood. Later I had the walnut example on loan for a couple months, and found it so close to the rosewood that I would been happy to keep it. Now, my friend had controlled as many variables as he could and paid particular attention to detail. No factory-built guitar is going to get that kind of attention, and the result is a much greater instrument-to-instrument variability within a particular sound-and-feel envelope--which is why Guilds tend to sound like Guilds and not Martins or Gibsons. But even the characteristic sound of a brand is going to fall into a range.
