If a single vowel is sung over multiple notes it’s usually good technique to keep the vowel sound consistent through the passage. (Unless it’s for intentional effect, aka rock and blues). One of the factors between a mediocre and professional chorus is consistent pronunciation throughout the group.
To sing a closed locked vowel like the “eee” sound, prepare the back of the throat as if singing an “ah”, but keep your lips in the pronunciation of the “ee”. This will allow the voice to open up through the higher register while keeping a consistent vowel sound.
In difficult classical passages for sopranos and tenors it may be necessary to slightly adjust the “ee” to more of an “ah” sound. The deciding factor is to keep the vowel sound as consistant as possible without negatively affecting the vocal tone. Only use as much “ah” as is necessary for good tone.
This helped me extend my range by a whole step. Thank you!