This question is better than some of the others. It's a shame nobody answered it. You seem to be a serious, thinking musician. You have serious musical reasons for wanting to change the original, so do it, even if the audience knows the music. (It's however probably a bad idea to do this in an exam or competition situation, where the judges may be too narrow-minded to appreciate what you're doing.) I don't think Liszt would be annoyed if a thinking musician wants to try something different with his music. I am sure he did that with other people's music (I read a comment that he played music better when sightreading it because the second time he would add embellishments).
Plenty of great pianists have disobeyed the composer's directions, e.g.:
-Chopin sometimes improvised when playing his own stuff in public.
-Horrowitz in 1965 played the ending of the chopin op. 10/8 as piano instead of forte.
In 1985 he changed the notes in the development of the 1st movement of Mozart K330, where he surely improves the original. And he says he deliberately played wrong notes to get certain effects sometimes (that's clearly disobeying the composer).
-Bolet said he changed the score if he thought the composer would have liked it (you can hear examples in the slow bit in his Liszt Harmonies du Soir).
-Cortot played the bass notes an octave lower in the Chopin funeral march.
-Gould and Arrau both disobeyed Brahms' tempo directions in the d minor concerto (at least that's what I read, I haven't seen the score).
Ok, people sometimes accuse such performers of thinking they are better than the composer. But that's rubbish: they're just saying that they happen to have an idea which is better/just as good at a particular point in some piece.
Moreover, it's impossible for any composer, however great, to think of all the possible ways of playing a piece or all the possible different notes. So if you are thinking of deviating from what Liszt wrote, it's impossible to know if Liszt had thought of your alternative and dismissed it. So you'll never know if Liszt would have disapproved of your alternative, even if you're playing p when he wrote f.