Huh. That's a little silly given the venerable HSTORE already has this capability:

      create table foo (d hstore);
      insert into foo (d) values(hstore(ARRAY['key', 'key2'], 
      ARRAY['value', 'value2']));
    --  "key"=>"value", "key2"=>"value2"
      update foo set d = d || hstore('key', UPPER(d->'key'));
    -- "key"=>"VALUE", "key2"=>"value2"

Seems like JSON / JSONB need operators to merge two objects. In fact isn't JSONB sharing the data format with the new HSTORE2?