Using a grindstone on a sincerely loyal trident will remove the loyalty enchantment, but it will not remove the nbt data related to its owner. Enchanting the trident with loyalty once more will make it sincere to its previous owner despite having a lower level of loyalty.
Using a grindstone on a sincerely loyal trident will remove the loyalty enchantment, but it will not remove the nbt data related to its owner. Enchanting the trident with loyalty once more will make it sincere to its previous owner despite having a lower level of loyalty.