The opponent that the three swords style swordsman Zoro defeated in Thriller Bark, Ryouma, legend says that he had cut down a dragon(竜) with his sword, that sword is now in the hands of Zoro. In Punk Harzard, Zoro has already killed a dragon(竜).
Now, Zoro and the Strawhats are in Ryouma’s homeland, Wano, to take down one of the four emperors, Kaido. Kaido is a dragon(龍), so perhaps the legend of dragon-slaying will happen again.
Dragon(竜) and dragon(龍): Similar yet different
Ryouma had a legend of killing dragon(竜), and now the country is guarded by a dragon(龍), Wise king Kaido. In the time of 23 years, dragon(竜) shifted from enemy to guardian, it’s not just a change of kanji, but perhaps the society as well. In current Wano, the idea of opening the border and the Kozuki clan that wants to do so are all considered evil. So perhaps it’s a political movement to hide the past by switching to “龍”.