ABC said:

What is the difference between a "change" and an "alteration" ?

There is none in common parlance. To change is to alter. You can describe different kinds of change if you want to do so, but to use one of a pair of synonyms to refer to one kind of change, and the other to refer to another kind of change, would be a purely arbitrary distinction.

And, while there is some validity to the notion that by altering the Hobbit he muddles the plot point in LOTR, realize that he does include a preface in the Hobbit stating that the story of the riddles used to be told differently, but the truth was discovered and so now it has been updated. It's rather clever how he explains the discrepancy in LOTR, and the change in the Hobbit preface.

Where does it say that in the preface? I have two copies of the book and in the preface of neither does this statement about the riddle-game appear. I have never seen this. Perhaps we have different versions?