You are correct. The Pro-HD has a DVB-T tuner so you can only watch SD channels on tuner mode. For HD you'll need to connect an additional tuner making sure its output is component as there's no HDMI in.
Or you can do as I did and use my old Humax PVR and use an upscaler to convert SD to 720p. Ok it's not true HD but I'm getting great results without investing in a new HD tuner as there's only a handful of HD channels on Freeview. To do this I have the upscaler and then an HDMI to component converter going into the Pro-HD.
I don't think a firmware upgrade even if there was one would be able to change the Slingbox to DVB-T2; but can anyone else answer this question?