JSF 2 AJAX declares "attributes" element that allows to update particular set of element attributes. However, to implement this feature, either updated component should be aware of this feature or full rendering & page content comparison with extraction of changes will be required. As a workaround, updates can be done simply via JS code.
Thanks Nick. I guess that a workaround is clear and it can solve this but it'll be very painful when you need to do this kind of re-rendering all the time.
P.S. I don't do such things =) it's only my thoughts about a4j.