[Guinevere]

In the Shire, Krege uses “Sie” instead of “Ihr”, which makes it sound more modern.
The hobbits mostly call each other „Du“, but then they mostly are friends or family. In the conversations in the „Ivybush“ a strange hobbit is addressed as „Sie“. The gaffer calls the miller „du, Sandigmann“ (but without „Herr“!)

The Sackville-Bagginses call Merry and Frodo „Sie“ („Sie sind kein Beutlin- Sie… Sie sind ein Brandybock!“) (in the old translation it’s “Du”)

Sam calls Frodo „Du“ and “Herr Frodo”, and sometimes “Du,Chef!” (Grrrr!) Gandalf he addresses in the beginning „Sie, Herr Gandalf“, later „Du, Herr Gandalf“, and finally Du Gandalf.

Farmer Maggot calls Frodo „Du, Herr Frodo“, but the Nazgul he tells „Verschwinden Sie!“
Mr Butterbur also calls the hobbits „Sie“ (and vice versa)

Strider and the hobbits also call each other „Sie“ until his declaration that he is Aragorn, and will give his live to save them. Thereafter they call each other Du. (in the old translation “Ihr” changes to “Du” a bit later, on the way to weathertop.)

In the Council of Elrond, Frodo calls Elrond “Du” (.Everyone is called „Du“, as far as I can see.. (Du, Meister Elrond)
Aragorn and Boromir call each other “Du” from the start. (In the Carroux translation it’s “Ihr”.) Even Sam says“Du“ to Elrond (“Aber du willst ihn doch nicht etwa allein losschicken, Meister?”)!!

Galadriel is called „Du“ by Frodo and also by Sam! ("Ihr" in the old translation)

Eomer and Aragorn say “Du” to each other. (In the old translation: "ihr")
But Théoden, as befits a king of old, is called “Ihr” by Gandalf, Eomer and everyone. (Also by Merry) (Théoden, on the other hand, calls Gandalf “Du”) (In the old translation Théoden calls Gandalf "Ihr")

At the first meeting with Faramir, Sam calls him “Sie”(!), and Frodo “Ihr”, but not for long. After the episode with the Mumak, both call him “Du” (and vice versa). (Old transl: both Frodo and Sam call Faramir "Ihr", Faramir calls Frodo "Ihr", but Sam "Du")

Gandalf calls Denethor “Ihr”, Denethor calls him “Du, Mithrandir”, but Pippin says “Sie” to Denethor (!) and he also says ”Sie” to Beregond, but only initially, after their snack on the walls, they have become friends and call each other “Du”. (Old translation: Gandalf and Pippin call Denethor "Ihr", Denethor calls Gandalf "Ihr" and Pippin "Du". Pippin and Beregond call each other "Ihr" the whole time.)

Aragorn and Eowyn call each other “Du” right from the start! So here there is no difference and hence no special significance of the passages where in the original “Thou” is used. (old translation:"Ihr", but "Du" in the passages where "Thou" is used)

Faramir and Eowyn also call each other “Du” right from the start. (old transation: "Ihr" the whole time)