If you have bubbled the wrong digits but have manually written the correct roll number in your KCET examination OMR sheet, then there is no need to worry as in such cases the OMR is always checked by a person and if any discrepancy is found, it is resolved in a manual manner and a person will fill your details manually.

So don't worry as your answer sheet will not be rejected and everything will be fine. Hope I have answered your question. All the best!