XBee firmware compability

We are about to start shipping XBee modules that acts as coordinators. We will after, say, tree years time, deliver a end node that is going to communicate with one of these coordinators. Is this possible without update the firmware in the coordinator (no new functions is used in the end node)

My question is: Can we use the latest firmware in every delivery, and still be sure that the XBee modules are compatible with each other (with reservation that no new functions is used).

Yes, the older firmware modules are compatible with newer firmware modules. It is always recommended to use latest firmware on all the modules, because the latest firmware will be bugs free and some will include new functionality.