Circuits in and out of the regional centres would need to be upgraded to carry HD, and you would probably need some form of upscaler.
Unless they adopt ITV’s approach and make the English regions remote sources into a centralised facility. Then no HD circuits into the region are required, and the SD regions could feed via SD circuits to the central point and be upscaled there. You lose the ability to soft opt from network and stick up captions and voice overs, but that can still be achieved up to a point with keyers etc
I believe that is the plan. It also has the additional benefit of not sending a network feed through regional centres 24/7 and could potentially reduce the level of resilience required (though since the move to co-sited coding and mux these may have been relaxed anyway)
As it's possible that the BBC are going to go for a remote production solution for their HD upgrades to the regional centres, putting in HD network feeds to each of them as a stop-gap is also potentially a waste of money (more in the I/O gear than the connectivity as I guess the move to BT provisioned circuits between centres means that HD contribution and distribution is pretty straightforward now)
harshy and thegeek gave kudos