Yes, Richard Adams did change the ending of "The Plague Dogs". In the original version, the dogs drown after swimming out to sea, a bleak and tragic ending. However, due to the publisher's and others' influence, Adams altered the ending to a more hopeful one where the dogs are rescued. The published version of the book features a happy ending where Snitter and Rowf are rescued by naturalists and reunited with Snitter's former master.
Here's a more detailed breakdown:

Original Ending (as envisioned by Adams):
The dogs, hunted by humans and pursued to the sea, swim out into the water. They are last seen enveloped in a mist, with their fate uncertain, but with the implication of drowning.
Published Ending:
The dogs are spotted by a boat, and rescued by Sir Peter Scott and Ronald Lockley, who happen to be in the area. They are then reunited with Snitter's former master.
Reason for Change:
Adams' editor and others felt the original ending was too bleak and upsetting for readers. They convinced him to create a more optimistic conclusion.

The film adaptation, however, returns to Adams' original vision of a tragic ending.