% assuming I was your input image
X_newInPlace = zeros(size(I));
CA = X_newInPlace(1:2:end,1:2:end);
CH = X_newInPlace(2:2:end,1:2:end);
CV = X_newInPlace(1:2:end,2:2:end);
CD = X_newInPlace(2:2:end,2:2:end);