My advice is to read
The Lord of the Rings first [before reading any responses here or elsewhere], especially as you plan to read the books soon in any case.
I know that's not very helpful but opinions about differences or 'faithfulness' vary widely, and a mere list of changes (even if somewhat subjective as to what constitutes a 'change' or what is important), will possibly contain
book spoilers.A long post? I guess it's relative