It's not as if there isn't already quite a bit of carbon fiber in the DA40, if that's the primary expected salvation. The main difference between the DA40 and 42 looks to be in the wing spars (DA40 is part CFRP, DA42 all CFRP) and main fuselage (DA40 is GFRP, DA42 is all CFRP, which precludes internal antennae, FWIW). Wing skin, control surfaces, and horizontal stabilizer look to be of similar construction for the DA42 and DA40. And it isn't as if carbon fiber is completely weightless.
Changing the materials used in the landing gear would yield some savings. But I've held a main gear leg in my hand and they aren't all that heavy. Fairings are CFRP. The use of Cleveland wheels and brakes has the practical benefit of their being ubiquitous and inexpensive to work on.
There are other areas if you're willing to go there:
1. Ailerons each have 4 hinges.
2. Depending on how you count them, the flaps have 6 or 7 hinges apiece.
3. DA40 has internal aluminum fuel cells. Many other aircraft have some kind of wet wing. Not necessarily the best idea in a composite aircraft. Exhibit A: Cirrus.
2002 DA40: MT, PF, 530W/430W, KAP140, Surefly, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream.