If 7D2 is going to be full frame so yeah going to upgrade with those rumored specs!
IMHO, the only way a FF sensor on 7D2 would make sense, would be If they also gave it a crop sensor mode. Something to allow current APS-C shooters with lots of EF-S lenses continue to use their glass.
that is not possible with the EF mount and EF-S lenses.... or is it?
Well 7D to 1100D have already EF and EF-S mounts so why not?
they are not FF.
the APS-C sensor has to cover a smaller image circle.
you can adopt nikon lenses on canon cameras but not the other way around (flange distance). so it´s not always a bi-directional compatibility.
The "S" in EFS stands for "Short back focus", which means the distance between the rearmost optical surface of the lens and the film (or in this case the digital sensor) is shorter than that of a normal Canon EF series lens. A shorter back focus does have some advantages when designing wide-angle lenses, but there's a limit to how short it can be in an SLR lens since it can't be so short that the SLR mirror hits it when it flips up. The lower limit on the size of the mirror depends on the size of the film (or sensor). Medium format SLRs need a big mirror, 35mm SLRs need a smaller mirror and SLRs with a digital sensor smaller than a full frame 35mm frame can use an even smaller mirror.
Since the sensor in the Digital Rebel is only 22.7mm x 15.1mm, smaller than the 24mm x 36mm full frame 35mm frame size, the SLR mirror can be (and is) smaller, so it can use a lens with a shorter back focus than full frame cameras, hence the EF-S lens.
Close, but EF-S lenses will not work on FF cameras for another reason. In the Canon system the EF-S stands for short back focus, meaning that the rear element is able to protrude further into the camera body than a 35mm lens. This is only allowed by decreasing the size of the reflex mirror.
In the Nikon system, they kept the mount and specifications for intrusion the same for their 'DX' lenses. Therefore, you can mount any DX lens onto an FX camera (full-frame). The only difference is DX lenses will not project an image on all parts of the FX sensor due to the smaller image circle. The D70 is an example of a Nikon camera with a mirror the same size as a 35mm film camera, despite having a smaller sensor size.
If you tried to mount an EF-S lens on a full-frame or 35mm camera, the reflex mirror would either prevent the lens from mounting, or would break the mirror when the shutter is pressed