Prime Focus Feed using with 5m  TX in Ku Band

Most cassegrain antennas have 'shaped' reflector surfaces. The subreflector is not truly hyperbolic and the main reflector is not truly parabolic.  The purpose of the shaped subreflector is to distribute power more evenly across most of the area of the main reflector while keeping the edge illumination low.  The idea is to get a combination of the best on-axis gain while still meeting the off-axis sidelobe envelope requirements. The shaped main reflector helps to get the main beam phase front flat across the entire dish aperture, which means it is in focus and has high gain.

If you replace the existing cassegrain feed with a new one the important considerations are the distance from the phase centre of the feed to the subrefector, the illumination level at the edge of the subreflector and the cross-polarisation characteristices of the new feed.

The idea of removing the subreflector and using a prime focus feed will work only if the main reflector is actually a true parabola, as is the case for smaller VSAT and TVRO antennas.  You need a prime focus type feed with a wide radiation pattern giving about - 16 dB illumination level at the edge of the main reflector and with good cross-pol characteristics.

Whatever you do, your biggest problem is likely to be getting the new antenna to meet the transmit sidelobe and cross-pol specifications, which are rather important if you are going to use the antenna for transmitting TV.

Seek more advice from the original antenna manufacturer and get help also from reputable suppliers of feeds (such as Vertex or ViaSat).  Contact your satellite operator and ask for full details of their requirements for transmit earth station cross-pol and sidelobe specifications and the test methods you must use to verify complianace.

Read my advice about how to adjust the subreflector on a cassegrain antenna.
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Best regards, Eric.