It may be true that you have expanded your higher voice range. However, are you also able to sustain these newly-acquired high notes? It can actually be unhealthy if you try singing a song that has a lot of high notes.
This situation is actually a tessitura problem. The tessitura is a voice range which is comfortable to you and wherein you can sing the notes on-pitch and without feeling strain in the vocal chords. This term is also used in describing the average or median pitch range of a song or chorus.
A lot of mezzo-sopranos can reach the higher C but their tessitura is actually an octave lower than that. Therefore, in cases where they sing high G to high C, they may experience vocal fatigue or strain.
Do not fret, however, because it is very possible to raise your tessitura. It definitely takes some work but it is surely worth it. Breath support with the upper resonance is what is needed for this task. To reach high notes, it is very crucial that you know how to control your breath because if not, the result will be strain. If you repetitively do this bad habit, the effect can be permanent damage.
It takes more effort to sing higher notes. You will have to use all of the muscles needed for breathing like the abdominals, the spinals and the diaphragm while expanding your midsection at the same time. Your midsection must still remain expanded as you exhale.
Once you have worked out the breathing part, you can now focus on the upper resonance or how to use your head voice. Instead of thinking that the notes are horizontal like in the music sheet, try to think of them as vertical. It is as if you are riding an elevator and your breathing is the cause for the elevator moving upwards.
When using the head voice, you will be feeling vibrations in the nose area and in the soft palate. Keep the mouth narrow horizontally but vertically tall. It is also important to keep your cheeks high and at times, your eyebrows for more head space. It is almost like you swallowed something you do not like and you are trying to keep it from touching the sides of your inner cheeks.
Do not force yourself to produce a high tone right away and remember to keep the lightness of the tone. Inhale deeply and almost like yawning, release a “hee” or “hoo” starting from your highest note down to the lowest. After that, this time, repeat the process but start one note higher.
Another exercise would be the vocal siren but this starts from the bottom of your range going up. This is a humming exercise that when your breath gets stronger, try doing it on the same breath.
Well, with the right practice and lessons you will definitely achieve the range that you aim for. Just remember all the techniques so that you will develop healthy singing habits.