Just that they updated the translation and started calling her by what she would have been called in FFVII if they didn't make the mistake in the original by taking the Japanese pronunciation as literal. She was always called Aerith, regardless of how it was spelled in the script.
Aeris doesn't exist, not in English or Japanese, as technically saying Aeris isn't even how it would be pronounced in Japanese, close, but not the same. In English however.. well, its just not her name.
If someone misspelled my name and started calling me by that misspelling it doesn't suddenly become by name. Otherwise i would have a whole host of variations over here.