I'm sure someone else must have already raised this idea, though I haven't noticed it: I wonder if we can/should think that the teaser version of the ship was the original configuration of the Crossfield class, from which Glenn and Discovery were refitted? Discovery is suggested to be "brand new" and "designed around" the spore drive, but this could be in the same sense that the NCC-1701 in TMP was "an almost totally new Enterprise" built around "redesigned" engines that hadn't yet been tested. And that might explain why Glenn and Discovery are/were the only ships of their kind, despite neither being the class ship Crossfield. In other words, the revamp of the ship that took place out-of-universe also took place in-universe. Thoughts?
That's exactly how I see it (except I think the unmodified Crossfield looks more like the version of the ship from the leaked test-shots, not the teaser, because it's considerably less hideous).