DMSS is JR's entry into the US market with 2.4 GHz, not related to Spektrum.
DSM/DSM2/DSMx are Spektrum algorithms, and have been shared with the current batch of JR radios.
Spektrum most likely will continue down the DSM-based path, while JR will go the DMSS route.
Whether or not JR will continue its collaboration with Spektrum remains a question.
DMSS equipment will not be compatible with DSM-based equipment, nor will DSM-based equipment be compatible with DMSS equipment.
No, you don't need to replace all your existing receivers, unless you also plan on replacing your DSM-based transmitter. If you plan on switching to the XG7/8/11 transmitters, you will have to switch out all your existing DSM-based receivers.
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