HURR HURR I"M DUMB SERIOUSLY WHY DIDN"T I SEARCH BETTER:
Answer (for anyone that comes up on this thread and wants to know)
To change the application launched by the Today screen Left soft key:
HKCU\Software\Microsoft\Today\Keys\112\Open = "\Windows\Calendar.exe" (REG_SZ string, including quotes)
To change the title of the Left soft key:
HKCU\Software\Microsoft\Today\Keys\112\(Default) = "Calendar" (REG_SZ string, no quotes)