XBee PRO XR Communication with XBee PRO SX

I have the discontinued XBee PRO SX radios that are currently running on the legacy firmware version XTC Hopping. My goal is to get an XBee PRO XR radio to communicate with the deprecated SX radios. Is there firmware that I could flash to the XR radios so that they are compatible? If not, why? What is the fundamental difference between them?

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