The 2H or AB structure may be transformed to 6H or ABCACB structure if deformation faults occur preferentially after every third close-packed layer. The theory of X-ray diffraction from one-dimensionally disordered crystals undergoing the 2H $\rightarrow$ 6H structural transformation by such a deformation mechanism is developed. For this, it is necessary to consider that the faults are not distributed entirely at random but tend to occur in such a manner as to statistically create a 6H structure. An exact expression for the diffracted intensity for crystals undergoing the 2H $\rightarrow$ 6H transformation by the deformation mechanism has been obtained and the different observable diffraction effects have been predicted. These results are very different from those obtained for 2H crystals containing an entirely random distribution of deformation faults, especially for large fault probabilities.