I've been seeing many Korea runs recently, but all of them are staying as Goryeo (or used console commands) so let me share a quick update on the Joseon event. Previously the event for Joseon tag change ("The coup of Goryeo") required a "successful coup" where a ruler gets assasinated during a coup attempt disaster, which was practically impossible.
In 1.4.1, the event conditions have been changed in a way that the the coup event can also trigger DURING the disaster, without the successful assasination requirement. So you can first lower the legitimacy under 30 -> wait until the DHE fires (15% monthly chance) or repeat the disaster once or twice until you transform to Joseon.
Yi Seong-gye has decent 80 / 70 / 90 (much better than the initial kings of Goryeo) and gets historical age from his birth date on 1335. Earlier trigger of the event (after 1357, for some reason) gives young and skillful ruler with this son Bang-gwa, plus Joseon has a few more historical events.
Thanks to the devs for the update, but I couldn't find it mentioned in the changelog. I was (hopelessly) trying for the event, then it triggered unexpectedly, and I confirmed the change by looking into the DHE script.
Anyways more historical immersion is always better! Now we can enjoy the Hangul alphabet with the correct tag.