I need to go BWI-HOU soon. There is a $320 fare now with rules I can live with.

I am in a position with this trip to have some extra time so I'd like to rack up extra miles

The routing is BWI-ORD-HOU and back HOU-ORD-BWI. Can I book this then will they let me reroute BWI-DEN-HOU and HOU-DEN-BWI? If I look at this routing now it is very expensive but once I book the cheap fare through ORD will they let me change?

Thanks!