Both Sephirot and Sephiroth are correct "English" transliterations of the Hebrew word, the plural form of Sephirah.
Here is an image to show that the original concept name was "セフィロス," Sefirosu, not "セフィロト," Sefiroto.
https://vignette.wikia.nocookie.net/...20121231151609
As you said the spelling change was most likely intentional, but probably to avoid another issue similar to what happened when the Crystal Tower was being released, the issue with "タイタン" vs "ティターン" as both would/could be Titan to an English audience; Audial cues for western languages vs Visual cues for Eastern languages.
The image might open too small to see you just click on it in the wikia page and you'll be able to see it better.
http://finalfantasy.wikia.com/wiki/F...al_Fantasy_VI)
