There are various sources (Chazon Ovadia (Yamim Noraim, p. 83), Yalkut Yosef 56:25, 101:4-5, Shu"t Ohr LeTzion (vol. 2 Perek 5 Teshuvah 11, Perek 7 Teshuvah 38) that say that Sephardim should not do certain things according to the Ashkenazi custom.
In light of the above sources we will explain what our sages mean when they say that a Sephardi may not change his pronunciation of the words to that of the Ashkenazim, i.e pronouncing the letter Thav as Sav or not stressing the Ayin or saying Shabbat as Shabbos etc.
It is known and widely accepted by all scholarship that Sephardic tradition has roots going back generations and should not be shirked, even when praying with Ashkenazim. Scholars add that Ashkenazim may, however, choose to pray in the Sephardi pronunciation.
Interestingly, the Chida explains on the subject of Tefilla that the Sephardic Nusach is laden with more Kabbalistic secrets than the Ashkenazi one, and, according to the Arizal, has more routes to Heaven. Therefore, many Ashkenazim who hold Kabbalistic customs can switch to Nusach Edot HaMizrach.
In general, if there is a communal place in which the community prays there should be a separate Minyan for Sephardim to pray according to their tradition, especially for Yamim Noraim.
For those living in isolated areas please consult a trusted posek.
Shavua Tov and Shabbat Shalom
