They say that if you don't use it, you lose it. For singers, if you never sing roles with high notes, you will never really have high notes, no matter how high you vocalize. Generally speaking, if you don't challenge yourself to do what is possible, but difficult, you may never fully solve your vocal problems. It is always easier for a soprano with problems to hide out as a mezzo: in my case, it is easier for me to hide out as a low bass, than to really fix my upper register.
Recently, I accepted an Opera by Request gig, to sing the part of Phanuel in Hérodiade, which is quite high for a bass. This part is written with a lot of exposed, dramatic D's E's and F's. I just can't sing it unless I do it right. Keys for concert work can be adjusted, but for opera you really have to step up to the plate.
What I realized today, is that you have to open for the notes over the primo passagio (for me an A) in a very high, frontal place. When it feels right, it is almost like opening up and behind the nose. This is the lift of the palate so crucial for the upper register. In my case, until I started to use a neti pot, I was chronically unable to breathe through my right nostril. The feeling of 'deadness' in that area meant that I was not able to open fully for the upper register.
You can hurt yourself singing high notes too 'open'; that is, with an uncovered approach to the vowel and the larynx high. Just modifying the vowel, however will still not give you the secret to singing those notes. You have to have the courage to open in the right way. "Covering" the vowels with a tight throat just gets you the unpleasant pharyngeal sound and an uncomfortable feeling of pressure in the throat. The support, or body response to the pitch also must play its part (more on that another time).
Open up and be safe! (When I get a chance, I will post an MP3 of me singing some of Phanuel, and hopefully, it will be the "right" way.)