This hair color and hair texture change seems to happen quite a bit with doodles and occasionally with poodles. Something happened to irritate the hair follicles. The source of irritation could lots of different things: a skin infection, a bug bite, hair dryer too hot, hair got tugged during rough play or rough grooming, or even a skin condition such as seasonal flank alopecia. Basically, anything that could trigger a little bit of inflammation around the hair follicles. The irritation triggers the hair to grow darker and coarser. Usually it doesn't seem to cause the dog any distress. If and when the hair returns to normal varies depending on what caused the problem and if the hair follicles are permanently damaged. It might be worth taking the dog to a vet with dermatology experience just to rule out an autoimmune issue, but I wouldn't say the problem requires you to clear your calendar and run the dog to the ER.