As stated in this digi page is possible to upgrade/convert a Xbee Series 2.5 to a Xbee ZB (with full standard Zigbee capabilities). Xbee Series 2.5 is just another name for the forme Series 2, so the upgrade procedure is exactly the same.
The procedure if fairly straighforward
Remove ATmega168 from Arduino board
Insert XbeeShield into [...]
