ZigBee module compatibility
Actually the answer from hanu is not correct.

The XBee series 2 modules are ZigBee compliant (ZigBee Certification). What you called the "Digi stack" is only the application profile they use. ZigBee compliant means they use the official ZigBee stack but operate on a private application profile. If you configure your XBee Series 2 in API mode you can talk to devices in other profiles.

The only problem you might get is with Smart Energy devices, as they use a slightly different way of communication, I believe it's because of more strict security requirements.

There are people who have been able to communicate to Home Automation devices of other manufacturers (connecting a HA smart plug). 
However you should keep in mind that this is not as simple as connecting a few XBees but there are libraries that can make it a bit easier.

A list of libraries for different programming languages:
http://examples.digi.com/quick-reference/

More links on that topic:
http://www.digieurope.de/support/forum/2384/xbee-pro-s2b-communicating-with-zigbee-ha-profile-sensors

EDIT:
Should you've meant one of the "old" series 2 models, their firmware can be upgraded. Here is the official guide from Digi:
Upgrading fromZNet 2.5 to ZB

Also:
 Firmware Revision History - XBee/XBee-PRO ZB Radio Modems