They did rename K-9. They just also have been making lots of improvements to K-9.

They still have two different apps, Thunderbird Mobile and K-9, but the only difference is the branding. All of the UI tweaks other than the icons and colours are in both apps. The functionality is identical. So other than not deleting K-9 it is a rename (I guess it is a hard link, not a rename).

That is just one of the downsides of the "stores" and basic package managers which do not have dependencies management where you can't do a transitional package.