Most of the song is in F Major, but in the chorus it abruptly moves to Ab. Since most people know the song very well, it is no longer a jarring change. But I imagine someone hearing it for the first time would struck by how quickly it moves to a new key and from tenor to soprano range:
Not only is it a note in a completely different key, the leap from "the" to "blues" is an (extremely) unusual minor-9 interval. That's not easy to sing even when it fits in one key. To singers struggling with this leap, I would recommending singing the "blues" note down an octave at first, so it is only a half step, then move it up. The second pass of the chorus only jumps from E to Db, but is still a difficult note to hit with no setup.
The Whitney Houston power ballad Don't Walk Away has an almost identical structure. At the chorus it moves to a key a minor third up, starting on the IV chord, exactly like Yellow Brick Road:
This time there is some preparation, as the Bb in the pickup notes establishes the new key. It is also a comfortable whole step into the new key, and the "landing" pitch is a common note between both keys. It's still a big moment, but perhaps not as jarring and difficult to sing as Elton John's key change:
The big moment in this song comes at the end when the song moves up a half step, in one of the all-time great modulations. She sings the three pickup notes then takes the final pitch up a half step, making for an awkward augmented second leap into the new key (at 3:40 in the video):
Such an epic moment! Trying to land on the Major 7 of the chord while reaching over an augmented step feels like tossing a book onto the top shelf and hoping it lands upright. There were so many different ways they could have made this modulation happen, and I love that they chose one of the most difficult. Try singing it without accompaniment if you doubt me.
For singers struggling with landing the D#, I recommend thinking of it as "Fa" in the old key, rather than "Mi" in the new key. I'd practice without modulating, playing an Eb chord instead of the Emaj9 for a while. If there's one saving grace about the modulation it's that the top note is in both keys.
Both are great songs, and both excellent studies in modulation. I never thought I'd write about Whitney and Elton in the same post. It wasn't easy to find commonality between the two divas, but believe I found it.