Copy an ImmutableMap with additional or new values

Much like how an ImmutableList could be extended as such:

ImmutableList<Long> originalList = ImmutableList.of(1, 2, 3);
ImmutableList<Long> extendedList = Iterables.concat(originalList, ImmutableList.of(4, 5));

If I have an existing map, how could I extend it (or create a new copy with replaced values)?

ImmutableMap<String, Long> oldPrices = ImmutableMap.of("banana", 4, "apple", 7);
ImmutableMap<String, Long> newPrices = … // Increase apple prices, leave others.
                                         //  => { "banana": 4, "apple": 9 }

(Let's not seek an efficient solution, as apparently that doesn't exist by design. This question rather seeks the most idiomatic solution.)

You could explicitly create a builder:

ImmutableMap<String, Long> oldPrices = ImmutableMap.of("banana", 4, "apple", 7);
ImmutableMap<String, Long> newPrices =
    new ImmutableMap.Builder()
    .putAll(oldPrices)
    .put("orange", 9)
    .build();

EDIT:
As noted in the comments, this won't allow overriding existing values. This can be done by going through an initializer block of a different Map (e.g., a HashMap). It's anything but elegant, but it should work:

ImmutableMap<String, Long> oldPrices = ImmutableMap.of("banana", 4, "apple", 7);
ImmutableMap<String, Long> newPrices =
    new ImmutableMap.Builder()
    .putAll(new HashMap<>() {{
        putAll(oldPrices);
        put("orange", 9); // new value
        put("apple", 12); // override an old value
     }})
    .build();