If I have had my navel pierced for a year now, should I worry about it migrating?

Migration usually happens the first 5 months of the piercing if it were to migrate. Since you have already hit the fully healed mark your navel piercing should be fine. Just make sure to always use good quality metal that has no nickel that can irritate your skin, good quality like titanium, 316L Stainless steel, real white or yellow gold and you'll be fine. Good Luck.