Answer:
At first, Odysseus and his men refrain from eating the cattle because they all swore an oath not to eat any cattle. The situation changed when they were on the island because they were starving and trapped there for a month. Don't forget that it was Eurylochus who convinced the men to eat the Cattle of the Sun: "it's better to die at sea from the wrath of the gods, he said than to die of hunger."
I hope that helps! If you need more contextual evidence, it should be in the book.
