If you only want to upgrade the dining plan, you can do that anytime up to 72 hours (3 days) before arrival, but if you do it within 2 weeks you might be subject to a $15 fee, apparently they charge that if you make any change to a package within that time.
If you upgrade only the dining plan, and don't change resorts or add days, you can do this without losing the free dining discount at any time.