The biggest challenge will likely be mounting the c-band feedhorn at the exact height as the KU fredhorn. If the feedhorn is not centered exactly as the previous feedhorn, it will not receive all of the reflected signal and the dish elevation angle would need to be adjusted to compensate.
The distance from the reflector to the feedhorn opening is extremely important and the placement of the scalar and type of scalar is even more critical. Changing the placement of either the feedhorn or the scalar will greatly affect the number of channels and the Signal Quality readings!
Moving the conical scalar even 1/4" forward or aft on my 1.2m aimed at 99w makes the difference of receiving 9 more channels!
The reflector has a wider beamwidth pick-up pattern. If there is an adjacent satellite using a similar frequency, it will interfere with reception of your primary target satellite. To minimize this interference, it may be necessary to pan the dish away from the adjacent satellite to minimize the interference. Yes, this will also decrease the signal received from the target satellite, but you are trying to find the best Signal To Noise (SNR) ratio comparing the signal from the desired satellite and that of the unwanted noise from the adjacent satellite. This will vary between transponders, so it is likely that one transponder will have better Signal Quality reading in one position and another transponder will have higher Signal Quality reading at another angle. Oh, the joys of mini-BUDs!!!
The main reason the combined C/KU LNBFs do not work well is because the feedhorn cavity is actually tuned for C-band frequencies. The KU-band waves bounce around in the mistuned cavity and whatever remains of the signal is caught by a KU probe at the end of the tube.