D* has plans to add most of the remaining local markets in SD before the transition. Hopefully when this happens you can upgrade to their new DVR, and no longer need to use the antenna. This would run $99 or less, maybe free.
If not, you can upgrade to their HD-DVR, which can use the digital signals from your antenna.
That would run $259 with the anteanna adapter.
The free converter boxes won't work well in your situation, they won't integrate with your existing DVR.