Don't discard anything! As far as old I-94s go, anyway.
I would suggest including copies (front and back) of both your I-94 from the POE AND your I-94 from the approval letter. If you noticed, you will have to provide your entry information in Part 3 of I-485 - that's where your old I-94 will come into play, since the new one can't confirm information like Place of entry and Status of entry.
I don't think it will create any confusion - surely, you're not the first person to have changed status since last entry. You can include a separate sheet of paper with explanation, if you like.