The so-called "D-7" in season 1 was actually intended to be called a Sech class, that's what it's described as in the concept art and in all materials CBS gave to Eagle Moss for their model. The problem was someone in studio got a bit overzealous about continuity and told the computer voice actress to call the ship a "D-7" not realizing a completely new design was going to be used. Eagle Moss, for their part tried to explain things by saying that particular ship was using a tractor beam salvaged from a D-7 which is why the shuttle's computer identified the ship as one, but I guess that's out the window now that the D-7 is a new design currently being built.
one fix to all of this is to say that the D7 designation had more to do with powerplant/warp core arrangement than with specifics to the hull. It's a weak arguement, but my suggestion assumes that much of what we saw on the previous "D7" in season 1 was just bling and cladding related to whatever house owned or built it. Much as we've not seen how many different Constitution class variants?
That could have meant 24 different D7 type vessels. The new government is paring down and standardizing to a previously common look while the fleet is rebuilding and repairing.