I got myself some tefillin to start fulfilling the mitzvah. Thing is I am ashkenazi and got myself sepharadic tefillin and changing them is not an option. Should I use them following the ashkenazi customs (which would be hard because of the knot positions) or should I follow the sepharadic customs (i.e. wrap outwards, etc.) when wearing them?
ב"ה Shalom, First of all, for the sake of clarity to you and other potential readers, my understanding is that when you write to Sephardic Tefillin you are referring to the "parshiot" of parchment written by the Sephardic custom. Also by the Sephardic custom you are NOT referring to "Nusach Sefard" which is the Chassidic custom. Indeed, there are fundamental differences between the Ashkenazi and Sephardic customs in regard to the writing of Tefillin. Therefore, everyone to begin with should follow their family custom. In your situation in which you have Sephardic written Tefillin, you can fulfill the Mitzva of Tefillin with a Bracha. שו"ת אגרות משה או"ח חלק ה סימן ב, שו"ת שו"ת יביע אומר חלק ט - אורח חיים סימן קג, יחוה דעת חלק ב' סימן ג', חלק ד סימן ג, פסקי תשובות אורח חיים סימן לו אות א ) However, according to Rav Ovadiah Yoseph zt"l (in the sources above) though for an Askenazi it poses no problem to put on Sephardic Tefillin, one who follows the Sephardic custom, the Askenazi Teffilin are Pasul= disqualified for use for him. I don't know what you mean by " and changing them is not an option". As I said, in the, meantime, you are fulfilling the Mitzva. However, in order to change the writing, you don't have to change everything only the parchment which is not the most complicated thing to change. In regard to the knots you should follow your Ashkenazi customs so at least in regard to the straps and knots you will be following your custom. All the best