Единственное, что нашел (все ссылаются на то, что изменил, но не приводят ссылок, типа общеизвестный факт)

https://structomagazine.co.uk/interviews/richard-adams/

и цитата оттуда, кто не хочет читать все целиком

В: Well, you won that argument! While we’re on the subject of changes, the first edition of The Plague Dogs ends when they’re swimming out to the island. In subsequent editions they are picked up in the boat. Whose decision was that?

О: I wanted them to have a happy ending. To have them just swimming out was almost a tragic ending, you felt that perhaps they might not do it. Didn’t they say, ‘I can’t go on Rowf, I can’t go on.’ Snitter says, ‘I can’t go on.’ I think that’s about the last thing [in the first edition]. Well, I wanted a happier ending. I can’t remember why I didn’t put it in to begin with… Well, never mind, but it was under pressure from various fans and supporters that I decided in the end to have a happy ending, and it struck me as a very nice idea to have them rescued by Sir Peter Scott. Did you like that? Well, my wife didn’t like that so much; she thought it would be good if they were rescued by a group of children who were having a little run out to amuse themselves. That would have been OK, but I’m quite content with it being Sir Peter Scott. I didn’t ask Sir Peter Scott before I did it, and somebody told him, and he said, ‘ah, and how many girls do I sleep with?’ [Laughter] Later on I met Sir Peter Scott – great chap of course, splendid chap, as everybody knows.

They made an animated film of The Plague Dogs as well as Watership Down.

They did.

And they kept the original ending for the film. Was that a decision on the part of the filmmakers, or did they make that as the book was released?

It was entirely them and not me. I would have put in the happy ending. I don’t know why they didn’t, they never told me. Each way it made quite an effective ending I think, it depended how you wanted to look at the thing. I was very keen on The Plague Dogs, keener on that than of any other thing other than Watership Down. The idea behind The Plague Dogs took hold of me very powerfully, so that I was meditating it wherever I went. It was on my mind all the time; I carried a little book to make notes for the book.