Can I change the S-box and P-box in DES?
S &amp; P boxes are not random permutations.
Changing them is not easy. 
The position of each element of an such a box is chosen following principles such as Strict Avalanche criterion, Bit independence criterion, non linearity, xor table distribution and maximum expected linear probability.

Not following these makes it prone to differential cryptanalysis.

See Biham &amp; Shamir's work on the cryptanalysis of DES S-Boxes.

Also check out "Comparison of random s-box generation methods" by Dragan Lambić for a different perspective.