There is a way to send HD over Coax but it is very expensive. I very often use HD Modulators to achieve this and it is very good indeed, I feed via a std RG6 coax many TV's, think 186 is the most so far, in a Hotel. The Picture Quality is superb and the distances you can run are almost limitless, again I have fed almost 1km from the Modulator to the furthest TV.
As an option, I would look into Bell TV, as the Sat provider, so you only need the one RG6 that is there now and then what about considering sending the HD Signals via a Wireless system to the TV's.? Think I have used Brite-View in the past, with very good results.