In the real world, altering, editing, rearranging and/or adding to other artists' work is standard practice. Ask any Hollywood script writer. If it were taboo to change a script after it was written or published, movies would never get made. Some of the greatest films ever were the result of a script being passed from writer to writer, transformed all out of recognition.
"Casablanca" originally was supposed to be a musical called "Everybody Comes to Rick's," starring Ronald Reagan and Anne Shirley! "The Maltese Falcon" was filmed twice (in 1931 and 1936) before we got the "classic" 1941 Bogart/Astor/Lorre/Greenstreet version. Charlie Chaplin took his 1925 "The Gold Rush" - a finished film and a classic by any definition - and in 1942, re-edited it, and added a score and narration. Many people regard this revised version as the true classic.
Getting back to our man Bix, you might recall the "Great Day" incident in 1929: Bill Challis sold a reluctant composer Vincent Youmans on the idea of allowing Bix to improvise a chorus on the upcoming Whiteman record of this song. Youmans was a genius and a consummate craftsman, whose songs are on a par with Gershwin's, Cole Porter's or anyone you can name. His music is perfect just as published. As far as Vince was concerned, "Great Day" was already a finished product. Well, due to circumstances beyond Challis' control, the solo went to Andy Secrest, and when the record came out, Youmans was LIVID. He might not have been any too happy with Bix's effort either.
When I first heard Bix's solos on Whiteman records, I couldn't imagine how they ever could be improved. They seemed to be the very template of inalterable perfection. Then I heard the alternate takes, where Bix shreds his original conception and plays something utterly different - and equally definitive. That pulled the rug out from under my notion of what is a "finished" work of art.
The point is that deciding when a creation is finished is more often than not a matter of opinion. The real question is: "Does it work?"