The HDNet Movies Channel will be showing Rio Grande in High Definition throughout April and into May. The first broadcast is April 10 at 6:30 p.m. EST.
Go here for the schedule:
http://www.hdnetmovies.com/schedules/
Put "Rio Grande" into the search engine at this schedule page and you'll come up with the schedule for the film.
I'm really looking forward to seeing Rio Grande in HD!