dnd 5e – How do a changeling’s Change Appearance and a druid’s Wild Shape interact?

The rules are somewhat unclear on this. While it is rather clear that basic shape was meant to refer to the changeling’s, the part you cite somewhat contradicts this, by suggesting that a transformation quadruped -> quadruped works, especially so, since a changeling will by default never be a quadruped.

I’d rule against the latter because RAI seems clear to me. If you rule otherwise, there is no reason to restrict the possibilities to wild shape forms so long as shape and size conform to the changeling’s rules, such as one large quadruped to another one.

The CR is irrelevant when using change appearance

However you decide, the changeling’s trait does not change any mechanics, only the appearance. If you decide that quadrupeds can be interchanged, you can transform from a riding horse to an owlbear for example, but the statistics then are still those of the horse.