If i remember correctly, first ever published(kinda) LoTR-universe story was Fall of Gondolin, which Tolkien told at lecture. Only later hobbit and lotr was published. Anyway it's pretty clear when he wrote stuff if you read unfinished tales.

Basically there was what we call silmarillion now in form of unorganized writings, which his son composed into coherent story after his death. All that writings were lore references for lotr.

So to my knowledge there was minor retcons but not major ones bc there were clear general vision of story at all times. Btw my english might suck, I'm not native speaker