The revision in the name occured when the adaption of the High German dialect, which was spoken in the more mountainous and populous southern part of the newly formed (to the Grimms) independent German federation, took over from the northern plains speakers of the Low German dialect (which the Grimms apparently learned early on).
At present, there are over 80 million speakers of the High German dialect, compared to around 3 million Low German speakers. It's been said hearing the Low and High German dialects is like hearing a native from Minnesota or the Dakotas and one from the Deep South, like from Georgia or Alabama in the United States! ;)
Danke sehr (Thank you very much) und
auf wiedersehen (til we meet again)...
🇩🇪 Sneewittchen 🇩🇪