If you don’t use lazy loading, you have the load the new Address explicitly (as you had to load it explicitly (with Include, for example), when you loaded the Person initially):
context.Entry(myPerson).Reload();
// If the person refers to another Address in the DB
// myPerson.Address will be null now
if (myPerson.Address == null)
context.Entry(myPerson).Reference(p => p.Address).Load();
// myPerson.Address will be populated with the new Address now
If you use lazy loading, you don’t need the second code block. Nonetheless, you get a new query to the database as soon as you access properties of the new myPerson.Address (like you have a new query in the second code block above) because the first line will mark the navigation property as not loaded if the person refers to a new address in the DB.
This behaviour doesn’t depend on whether you have exposed the foreign key in the model class or not.
There doesn’t seem to be a way to call some single magic Reload method which would reload and update the whole object graph in one call (similar like there is no single Include to eager load a complete object graph).