Arda was once flat, but after the fall of Numenor in the second Age Illuvatar removed Valinor from the mortal world and made the world round. The Undying Lands would have stayed the same way it always was, it's inhabitants were immortal and there was little to no significant change there. The Elves were still allowed to travel to Valinor over the Straight Road/Path, this was/is a path which takes you out of the mortal world and does not follow the circular shape of the world, instead it takes one to the Undying Lands.
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