The one who made Sandai is Heitetsu himself, and he's the one who has Nidai now, so there is no need for a sword to break, it's all in the hands of Heitetsu and Zoro to exchange one for another specially since Zoro will end up having many people in Wano admiring him, Heitetsu is no different, he might be the one to suggest an exchange to begin with, and Zoro may agree considering the swords belong to Heitetsu to begin with (not to mention, it's a power up, so no complaint)