If you were to alter a passage in any piece, I'd place a good bet on them knowing what you've done. I mentioned their musicality because it's relevant. They're musical, and will have a lot of experience, therefore will have encountered so many pieces that it doesn't matter if they are on a syllabus or not, they'll still be familiar with the piece.
For example if you did that Chopin Prelude in B minor for a solo, no it's not on the syllabus now, but it is such a popular piece, a lot of examiners will know it.