OC 687: EARLY PURIM MORNING CONVERSION
One who converts Purim morning after dawn but before sunrise is not obligated in the previous nighttime’s reading of Megillat Esther. However, the convert is obligated in the daytime megilla reading, as one was a Jew by the time the obligation became activated (sunrise).
PURIM DAY CONVERSION
In spite of its reading being allowed all day, there is a dispute whether one who converts during Purim day is obligated to read the megilla. This dispute is based on halacha’s assigning each person their proper Purim date (Adar 14 or Adar 15) based on their location at sunrise on Adar 14. Thus, although a convert should be obliged to read, both due to the fact that the reading obligation exists the entire day (as in entry OC 25) and that one can be assigned the default date of Adar 14, the convert should preferably avoid the question that year by hearing the megilla and its blessings, instead of reading it and blessing God directly.
CONVERTING IN JERUSALEM OVER A THREE-DAY PURIM WEEKEND (PURIM MESHULASH)
As regards someone who converts in Jerusalem over a three-day Purim weekend (Purim meshulash), when the city’s Adar 15 Shushan Purim falls out on Shabbat: It seems clear that if one converts on Sunday one is not obligated in any of the mitzvot of Purim since one missed the holiday date, and that if one converts on Friday one is obligated in all of them since one is Jewish by the time the holiday date begins. If one converts on Shabbat during the daytime, a question similar to the one discussed in the previous paragraph arises, but the question has little relevance in practice since conversions are prohibited on Shabbat.
GOING TO JERUSALEM ON SHUSHAN PURIM AFTER CONVERTING ON PURIM
One who converts on Purim afternoon, so that one’s obligation to read on Adar 14 is debated, and then goes to Jerusalem (the city that is indubitably obligated to celebrate Shushan Purim Adar 15) might be subject to special rules.
OC 689, 692: OBLIGATION TO READ THE MEGILLAH
A convert is obligated to read Megillat Esther with its blessings just as one is obligated in all mitzvot that commemorate national historical events. See earlier entries OC 47 and OC 60.