The 2H or AB... close-packed structure may be transformed to 6H or ABCACB... structure if layer displacement faults occur preferentially on 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 layer displacement mechanism is developed. It is shown that it is necessary to consider that the faults do not occur entirely at random but prefer to occur at three-layer separations from each other in order to statistically create a 6H structure. The diffraction theory, as developed by earlier workers for 2H crystals containing a completely random distribution of stacking faults, cannot therefore be applied to the present case. An exact expression for the diffracted intensity from crystals undergoing the 2H $\rightarrow$ 6H transformation has been obtained and the different observable diffraction effects (like change in integrated intensity, peak shift, integral breadth, peak asymmetry) have been predicted.