I completed the expansion of my legacy system and it went off without a hitch - except for my surprise at the cost of cables. I bit the bullet and bought compression/stripper tools, as well as 100 ft RG-6 (2300MHz tested) spools from Lowes. I could probably have gotten them cheaper, but Lowes was convenient for me. Also, "tested to 2300MHz" is stamped on the cable jackets. So I have some extra cable and tools that I may never use again, but my system can support 16 tuners. I posted a block diagram of my setup in the attached pdf file, along with my final Bill of Materials. D* provided a free HR24 in exchange for extending my subscription for two years. They also offered to replace all my legacy (owned) receivers for free.
The only downside is that the 1st floor's OTA antenna input feeds directly into the TV, so those OTA-input-channels do not show up in the HR24 on-screen guide, like they do on my HR20. I can live with it though, since it's only a back-up when we have heavy rains in the Mid-Atlantic states - like now!