By the time the last AWD is commissioned the first will need to go into refit to get upgraded to either Baseline 9.x or 10 (whatever is available).
All said and done, we should have bought Flight IIA Arleigh Burke's straight from the US production line. We could have bought twice as many ships, better ships, and benefit from a large and established engineering and logistic supply chain.
Lot of national pride goes along with defense acquisition.
Brits bought Apache helicopters from us and then there was a big kerfuffle because they didn’t buy local. They backed down and put British built engines and electronics into them. Things never worked right.
Yes, the Burke’s are pretty refined and have most of the kinks worked out.
Also, if the wiki page is to be believed, the Hobart class is currently outfitted with the SM-2MR (medium range) SAM system. However, stepping up to the BMD mission means buying a bunch of SM-3s. (not cheap on their own) The SM-3 is noteworthy for the addition of the large MK72 booster. As I understand it, this requires upgrades and changes to the VLS cells to handle the extra amounts of rocket exhaust. More thrust, pushing more weight out of the cells means those plenums and chambers in there have to deal with (ie. vent) more extremely hot, corrosive, and abrasive trans-sonic gases... So there's some "body and fender work" - welding, ablative coating, etc. etc. work too.
If the cost of upgrading two AWDs really is going to be 4 or 5 billion, I'd say don't. Instead, keep the AWDs at baseline and buy a pair of Burke class DDGs. Apparently they are running about 1.8 billion a pop. They're a known quantity, no muss, no fuss. Basically just drive them off the showroom floor - or out of the shipyard as it were. Double the hulls, more than double the capability. Less cost - initially. Though you would take on additional cost in manning and operating costs.