What is the religious ruling on my refraining from eating grapes after I made a covenant with Allah to abandon them for fear of elevated blood sugar, especially since doctors have now advised me to eat all foods?

Whoever makes a covenant with Allah to abandon something that is not an act of worship, his covenant is like an oath (yameen). If he violates what he covenanted with Allah, he is considered to have broken his oath and must offer an expiation for an oath (kaffarat yameen), for fulfilling a covenant with Allah is obligatory. If the reason for the covenant ceases to exist—for example, if doctors instruct him to eat grapes after they were previously harmful to him—then the covenant is nullified, and eating grapes would not be considered a breach of his covenant.