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 Arduino Change *both *jumpers in XbeeShield to USB Connect Arduino to the computer using the USB cabke Download the ZNet 2.5 to ZB Conversion Kit here Start the X-CTU 5.1.4.1 software Select the proper COM port (should be Usb serial port (COMX)) Click on Test Query. The following dialog should appear indicating that X-CTU found the Xbee in that COM port and giving you some information about the Xbee module version, etc. In my case it was a ZB24-B with 1220 firmware Go to the Modem Configuration tab Download the ZNet 2.5 to ZB Conversion Kit here if you didn't do it earlier Uncompress the ZIP Select the “Download new versions” button. Press the “File” source button Select the xbee_zb.zip file to add the firmware set to X-CTU Back to the Modem Configuration tab. Select "Always update firmware" checkbox and select XB24-ZB (or XB24P-ZB if you have a PRO module) and the function set that you want for the module (router, coordinator, end device, each one in two differente flavor AT or API). In my case I selected XB24-ZB and COORDINATOR API. Check the product manual to find out about API and AT modes and the different firmwares for each set Press "Write" and X-CTU will start uploading the firmware to the module The firmware upgrade is complete **NOTE**: Make sure that the configuration for DIO7 is DISABLED as this XBee pin is connected to the RST in the Arduino. If you leave the default option (1 - CTS) random resets after a couple of seconds may occur+ NOTE 2: Be aware that putting XBee to sleep will also raise DIO7 and produce and Arduino reset. See this other post on XBee sleep and Arduino reset