just completed this game today and to clear it up:
starting a new campaign on normal after finishing it on hard DOES NOT OVERWRITE YOUR HARD MODE PROGRESS (i.e., MEDALS) and it works towards unlocking the difficulty specific achievements.
once you finish the campaign on hard, simply select new, choose normal and it will still keep track of your medals from hard mode. when you go to stage select and highlight whichever difficulty you choose, you will then see which missions you've already earned medals in that difficulty.
if, however, you want to be completely sure, copy your save file to another memory. i know i did and found out i didn;t have to when i finally figured out how the game's save system works.
i've also PMed the guide author about it and hopefully he will make the necessary edits.
hope this clears things up.