The "A Clockwork Orange Resucked" introduction is from November 1986. It covers the reasoning behind the shortened American edition[0], Kubrick's film, and his own feelings on the 21st chapter[1]. I think the final paragraph summarizes it well "Readers of the twenty-first chapter must decide for themselves whether it enhances the book they presumably know or is really a discardable limb. I meant the book to end in this way, but my aesthetic judgement may have been faulty" (Burgess xv).
[0] "I needed money back in 1962, even the pittance I was being offered as an advance, and if the condition of the book's acceptance was also it's truncation - well, so be it" (Burgess x-xi)
[1] "There is no hint of this change of intention in the twentieth chapter. The boy is conditioned, then deconditioned, and he foresees with glee a resumption of the operation of free and violent will. [...] The twenty-first chapter gives the novel the quality of genuine fiction, an art founded on the principle that human beings change. There is, in fact, not much point in writing a novel unless you can show the possibility of moral transformation [...] The American version or Kubrickian Orange is a fable; the British or world one is a novel" (xii)