We're currently shipping v1.54 in all RRC3 altimeters, and this does work with the RTx/GPS unit. It will not "need" to come back in, however if you did want to upgrade when the RTx/GPS system gets released you can send it in at that time for the upgrade. Specifically for RTx/GPS enhancements, v1.60 downlinks the altimeter battery voltage and also provides real-time reflective continuity and event status data. I'll be making the details available when the time comes, and I'm anticipating first production shipments in January.
The RTx/GPS will mount directly to the same holes as the RRC3 (when using the 3D printed GPS Brace). The 98mm RRC3 dual sled is pictured below and shows the optional GPS Brace mount. Alternative mounting is also possible if you purchase the RTx/GPS with a traditional flush mounted GPS module. While you can use 9V alkalines with the RTx system (runs ~1.5 hrs.), we're recommending 3.7V 750mAh LiPO batteries which will fit into the 9V battery holder (runs ~10 hrs.).
The RTx/GPS unit will also bolt onto any 54mm, 75mm, or single 98mm printed sled (when using the 3D printed GPS Brace) if you wanted to mount in a cone or stack 2 sleds together in an extended av-bay section.