This is not as obvious as it should be. We will be reviewing the signatures for RefetchEntitiesAsync. In the meantime, the code should look like this:
var list = new EntityKeyList(typeof(Person), new EntityKey[] { somePerson.EntityAspect.EntityKey });
entityManager.RefetchEntitiesAsync(list, MergeStrategy.OverwriteChanges, RefetchCompleted, null);
...
void RefetchCompleted(EntityQueryOperation e) {
// Do stuff
}