Democratic Peoples’ Republic Korea (DPRK, North Korea) changed the face value of stamps under a new postage order from August 1 Juche 91 (2002). The Ministry of Post and Telecommunications of the DPRK took a measure that those previously issued 91 postage stamps were overprinted with the new denomination to be used for a certain period from August Juche 91 (2002) according to the postage changed.
This was reported in the Korean Stamp Catalogue 1946-2006, Korea Stamp Corporation, Pyongyang, 2007, page 272, and repeated in later catalogues, so the 1946-2015 catalogue, page 312:
No reason was given why it was necessary to overprint that much of stamps, 91 different. The overprints go back to 1963 until 2002.
A new set of Definitive stamps were issued on November 20 Juche 91 (2002), with 12 different denominations, but neither from them is corresponding with the surcharged denominations.