Keep those SD recordings as is for best quality archival purposes.
It sounds like you would benefit from utilizing a PC to burn HD content from OTA sources. Blu Ray drives are quite affordable now, and can be bolted onto a very entry level desktop PC with 4GB RAM/Intel Core i3 chipset & video capture card (<$500). I would pop over to the PC-related CNET sub-forums on the particulars of how you can do this.
Question: are you *really* watching that many HD shows via OTA for which you would need HD recording ability(?)
We use OTA broadcasting which comes in through our DTV-PAL which we then watch on our Sony HD TV. We sometimes record programs on our DTV-PAL. I would like to know if anyone can recommend an HD DVD recorder that I can use to record programs I want to keep onto a DVD. I currently use a Panasonic DVD recorder/VCR combo to record my programs onto a DVD, but the quality suffers greatly because it doesn't record in HD. (As a side note, I still have family videos on VHS that I am converting to DVD, so if there's a recorder that will upgrade the quality of my VHS recordings by using an HD recorder, I would be interested in that also). Thanks!