The old numbering schema was active for more than 35 years and transition could not be achieved over night - AMSS is still using the old schema. Look at those links:
http://www.amss.org.rs/index.php?option=com_google&view=standard&id=4&Itemid=181
http://www.putevi-srbije.rs/index.php?option=com_content&view=article&id=2860:130812-radovi-na-drzavnom-putu-ii-reda-broj-128-deonica-golubovac-donji-milanovac&catid=34:odrzavanje&Itemid=1632&lang=sr http://www.putevi-srbije.rs/index.php?option=com_content&view=article&id=2824:090812-obustava-saobrajaca-na-drzavnom-putu-ii-reda-r-267-ugrinovci-dobanovacka-petlja&catid=26:obustave-saobracaja&Itemid=1584&lang=sr http://www.putevi-srbije.rs/index.php?option=com_content&view=article&id=2817:080812-drzavni-put-ib-reda-broj-21-m-19-deonica-debrc-misar&catid=26:obustave-saobracaja&Itemid=1581&lang=sr
http://www.rts.rs/page/stories/sr/story/245/Servisne+vesti/1158734/Saobraćaj+pojačanog+intenziteta.html