It is not clear exactly what Dish will duplicate. With the massive spot beam capacity of both 61.5 and 129, they could do the larger markets on both. Either they do both, or they will have to go change out dishs in a lot of locations (like Dallas and Atlanta, currently installing on 129 but being duplicated on 61.5).
The duplicate SD vs HD only on channels is a mystery. It would be nice if they only uplinked the HD version, but perhaps it is contract obligations with the programming providers that make both versions needed. It will be interesting to see what is uplinked, if only certain channels are dupped, all dupped or none dupped.
In the case of the duplicate markets, it's simple economics. Dish will simply calculate number of subscribers to the package times the cost to repoint, and compare that to income they would receive from freeing up the transponder for other uses.
It is highly likely that
eventually all market duplication will end - because
over the next 5 years, Dish will need to add thousands of HD locals in a couple hundred markets.
In terms of the SD vs HD duplication, it seems certain that 72.5 will offer
all the National SD channels. The reason is that the point of the Eastern Arc is to offer a "one small dish solution". And, in the near term, many popular channels are still not HD (TV Land, ESPN Classic, MSNBC, Headline News, Comedy Channel, etc.), so those channels need to be offered to potential new subscribers.
And in all the HD Locals markets offered on 61.5, their SD Locals channels need to be on Eastern Arc, so those markets will be on Eastern Arc, and new SD-only subscribers will need Eastern Arc for the "one small dish solution".
This is why the Eastern Arc dish has three LNBs - to fit all those channels, you need three slots...