Jerry Beck has a side-by-side comparison of some shots from two versions of Tex Avery's cartoon The Shooting of Dan McGoo: the original 1945 version, and the 1951 reissue.
It's not the only cartoon that had something changed upon reissue; Bugs Bunny Rides Again had a Mahatma Ghandi joke redubbed, and Bugs Bunny's breakthrough cartoon A Wild Hare lost a joke about Carole Lombard. But I didn't realize that MGM would actually have cartoons re-drawn to eliminate topical jokes.
Another thing that leaps out from the comparison is how much better the still from the original version looks. Alas, as Jerry mentions, the negatives to most of the MGM cartoons were destroyed in a fire, so the second still is probably representative of the way most of the Avery cartoons have to look (except for the ones that survive in nitrate prints).
One other note on "The Shooting of Dan McGoo": Droopy is actually voiced by two different actors in this cartoon. His opening line (the ultimate Droopy line, "Hello, all you happy taxpayers") is voiced by the one and only Bill Thompson. But for the rest of the cartoon, Droopy is voiced by somebody else -- possibly Daws Butler, I'm not sure. (Thompson was the original voice of Droopy, and did the voice when he was available, but he wasn't always available, so other actors, including Butler, Don Messick, and Tex Avery himself, filled in in various cartoons.)