One of the more interesting modifications was Liszt's addition of the 4th movement to his transcription of Beethoven's Ninth Symphony.
There was a substantial gap between his first examination of the symphonies and the final version. Though he called the work "complete" with only 3 movements, Liszt later felt that the piano itself had improved to the point where it could represent the orchestral composition.
When Liszt began work transcribing the ninth symphony, he expressed
that "after a great deal of experimentation in various directions, I
was unable to deny the utter impossibility of even a partially
satisfactory and effective arrangement of the 4th movement. I hope you
will not take it amiss if I dispense with this and regard my
arrangements of the Beethoven symphonies as complete at the end of the
3rd movement of the Ninth." (He had in fact completed a transcription
of the Ninth Symphony for two pianos in 1850.)
Nevertheless, he made
another attempt after an expressive letter from Breitkopf & Härtel,
and expressed "the range achieved by the pianoforte in recent years as
a result of progress both in playing technique and in terms of
mechanical improvements enables more and better things to be achieved
than was previously possible. Through the immense development of its
harmonic power the piano is trying increasingly to adopt all
orchestral compositions. In the compass of its seven octaves it is
able, with only a few exceptions, to reproduce all the
characteristics, all the combination, all the forms of the deepest and
most profound works of music. It was with this intention that I embark
on the work which I now present to the world."
Source (paragraph break mine)
Another body of Liszt's work, his 12 Transcendental Études, underwent three revisions, including one that was intended to simplify them.