If memory serves, Drizzt's panther companion Guenhwyvar was referred to as a 'him' in the early Forgotten Realms books (Icewind Dale and Dark Elf trilogies) but a 'her' in later works (such as Starless Nights, Siege of Darkness and so on). Was this deliberately rewritten, or is my memory just faulty?
No: "he" was a publisher's error.
Quoting from here (emphasis mine):
In The Collected Stories: The Legend of Drizzt Anthology, R.A. Salvatore explained that while writing The Crystal Shard, he was told by TSR that magical items could not have a gender, despite his arguments otherwise. To his "horror," he later discovered after the book was published that some of the more awkward uses of "it" were changed — presumably by a copyeditor — to the male pronoun. Salvatore has subsequently responded to hundreds of letters from fans explaining this. Despite these publishing errors, Salvatore explained that Guenhwyvar had always been a "she."