In Kyon: Big Damn Hero, members of the SOS Brigade change the way they address each other as a symbol of their True Companions relationship is strengthening: Tsuruya is called Tsu-chan by Haruhi and Mikuru(big), and Yuki and Mikuru being called by their first names not only by Haruhi.
Used in the The X-Files / Earth: Final Conflict crossover "Apologia." The young Agent Sandoval asks if he can address Scully as "Dana," because he refuses to give the impression that he's trying to take Mulder's place.
A classic example of this trope appears in With Strings Attached when, as a sign of his complete Heel-Face Turn, the Hunter asks Paul to start calling him by his first name, Jim. He had already agreed to use the first names of the four that morning, rather than the insulting nicknames he'd come up with for them at the beginning. (Originally, when George said that they have names and would he care to hear them, he said “Not especially.”)
In the Katawa Shoujo fic "From Shizune's Perspective," Shizune temporarily switches to calling Emi by her first name rather than "Miss Ibarazaki" to make a point, then despite switching back to calling her by her last name, gradually switches over to her first name. She also points out that Hisao might be "interested" in Emi if he calls her by her first name, causing Misha to react in displeasure because she is in love with Hisao. Shizune also calls Misha by her first name, Shiina, in this fic, instead of her nickname.