Has anyone figured out if they are replacing the sensor. I don't understand how this can be just a firmware upgrade. Someone suggested the dual-pixel sensor had already been put in place in the C100, but that seems like really incredible advance planning to have put the sensor in a camera that was announced more than a year ago.
Well, the dpreview news blurb certainly mentions that the sensor will not be changed, but it's functionality will be activated. This certainly requires new firmware, but may also require additional new processing hardware. Note that the framerate for single shot is also increasing.
It's not surprising that the sensor was already there. Coming up with this dual sensor probably took Canon quite a while in development.
I speculate that the dual pixel technology was being developed for quite a while. It probably was meant to be deployed earlier, such as a new 7D II, the EOS-M, the C100, etc. The earthquake in Japan probably set back quite a number of projects. Anyway, when Canon felt they could no longer hold back on putting a mirror-less on the market, they put out the EOS-M with the "old" simple sensor-phase-detect AF used in the 650/700 cameras.
In case of the C100, by that time the sensor was already ready. However, for the AF, now that the sensor was available, they needed a lengthy period to develop the software (firmware) to allow the AF to work with all the lenses - and still not all work. They could release the C100, because that camera did not rely on working AF to be sold. In the case of the EOS-M, this approach would not have worked, since it needed a working AF, so the older sensor was used. By the time the 70D was released, almost everything was resolved. I say almost everything - because the 7D II is still to be released. If it would be just a question of better body etc., it shouldn't have held back the release of the 7D II, so I think there must be more going on with the sensor that they want to put into it (I'm still hoping for the high dynamic range trick, as also used by magic lantern)