The 920XT will, this month (Dec 2015), be firmware-updated to support the new running dynamics that were recently introduced for the 630 and even more recently promised for the Fenix3.
I’m assuming they will work with any old HRM-RUN and not just the special new HRM4-RUN. We’ll see.
This will also require a firmware upgrade to your HRM-RUN, so keep that handy when prompted to do a firmware update.
Don’t get too excited tho, the new special metrics are not that exciting really. And before you ask NO YOU DON’T GET THE PHYSIOLOGY measurement of LTHR, etc as found with the 630. That will probably be added in early 2016.