Logician, Rav Ovadia ztzuk”l said you can change from sfaradi to ashkenazi. That is NOT the same as what you say.
The current day Nusach Sefard is not Sefardi/oriental. It was made by the early Chassidim, around the time of the Ba’al HaTanya. The Ba’al HaTanya, took the tikunim that the Arizal made in davening and applied them to Nusach Ashkenaz, hence the name nusach Ari (used by Chabad). Later it was developed further and became what is known now as Nusach sefard, which is the tikunim al pi sod/kabalah which were previously only in sefaradi davening applied to nusach ashkenaz. It has nothing to do with the people known as sefardim.
So when litvishe tshuvos who say you can change from Nusach Sefard to Ashkenaz but not the other way around, it is because the mesorah of Nusach Ashkenaz is around 1000 years older than sefard. However, sefardi is probably just as old if not older than ashkenazi, and then you would not be able to change to ashkenazi.