When you're performing in front of people, your first responsibility is to the audience, then to the composer. So, if the composer wrote something that you can't play without diminishing the experience for the audience, then change it, leave it out, or something! (sorry, composers) Nobody likes to watch/hear someone struggling with their instrument. You have a responsibility to those people who have taken precious time out of their lives (which they'll NEVER get back) to come hear you play. No pressure...
Your responsibility to your audience also comes before yourself. I'm reminded of Rick Warren's immortal line: "It's not about you." Some performers--especially students--seem to put their own pride first by insisting on performing something that they can't really pull off technically. Ironically, they end up bringing more shame to themselves by trying to be macho. They could retain their pride by altering the music if necessary to allow for a polished performance, and almost nobody would even know they didn't play exactly what was on the page.
For example, there's this little ditty in the third movement of Mozart 2:
If you can't play this with all the trills by the time your recital comes along, then it's MUCH better to leave out the trills than to make a mess of it. There are no bonus points for effort in music!
If you can't gracefully play that low A-flat at the end of Nocturno, then take it up an octave rather than spoil the mood you created by playing the previous two pages beautifully.
Now, some very important caveats...
This post is not intended to let you "off the hook." Long before your recital, you should pick your repertoire carefully, so that you don't choose music that you won't be able to play well. Furthermore, you should do your absolute best to learn the music as written, and you should only change something as a last resort.
Of course, it's always good to pick a piece that will stretch you and help you grow as a horn player and musician. If you chose a piece in good faith, believing that you had a solid plan of attack to conquer all of its challenges before the performance, and you worked as hard as you could but find yourself still struggling with a note or trill just days before a performance, then you should exercise discretion (the better part of valor) and do what's best for your audience.
I'm curious if anybody feels strongly the other way?