Was the American Switch REALLY necessary?

The memoirs HAD to be read in the film in the American version, under the Code, to be sure that he would get his comeuppance. As others have posted, it couldn't be left ambiguous, or leave the audience thinking that he "got away with it" or it wouldn't get the film office's seal of approval. That's not to say that the film HAD to have the seal of approval (or whatever you want to call it) because there were plenty of post-code films released during the period the code was in effect. But there were plenty of distributors, theatre chains, film reviewers and so forth who would never touch a film not approved. Newspapers, magazine, radio & other media wouldn't publicize non-code films. And there was lots more competition for good box office in those days before television. There were plenty of other films out there getting big publicity and packing them into theatres, and an unpublicized film running in an off-brand theatre would have no chance. Even with the Code's stamp of approval, there were some films that couldn't overcome local civic or religious opposition at time. "Banned in Boston!" and "But will it play in Peoria" are two old phrases that come from the period...and actually, by the 50's & 60's, "Banned in Boston" was used in advertising in other towns as kind of a "nudge, nudge, wink, wink...you'll probably see bare boobs and lots of sex in this flick!"

OP: "The memoirs were going to be found, and he would subsequently be hanged. Was it really necessary to SHOW them being read? I mean, isn't the american viewing audience smart enough to realize what happens next without having it shown to them?"
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it's funny that some posters have called the hayes ending a "sledgehammer" ending. because i could say the same about the original ending. when he said at the end, essentially, "oh crap, my memoirs!" was it really necessary to SHOW the memoirs on the table? i mean, isn't the british viewing audience smart enough to realize what happens next without having it shown to them? you could take it one step further, and ask why he had to comment on his memoirs being forgotten, why not leave it to the observant audience to notice that he is leaving without them? one man's flyswatter is another man's sledgehammer.

as for other posters who used this issue to rail against american moralism, it seems a little late to criticize 1940's america. and if you are going to, there are much more important flaws to criticize than film editing. i think americans made up for moralistic endings with the usual suspects, primal fear, etc., just to name a couple that come to mind instantly.

Thank God I've never seen the American version!

It has happened in the opposite direction too. (US -> UK) In the last scene of VERTIGO a woman tells a detective about a man arranging a murder. The British insisted that Hitchcock add a scene specifying that the man had been arrested for it, as if the audience was too stupid to figure that out.

The memoirs were going to be found, and he would subsequently be hanged.