Okay, so I've beaten the game on Normal, and I collected all of the trading cards (even the secret 65th one). Now all I need to do is beat it on Easy and Hard, but I have a question about this. 
 
See, when replaying chapters (which I did a lot to do side quests I had missed), it lets you change the difficulty. Does anyone know if you can get those Achievements by replaying all of the chapters on different difficulties, rather than having to replay the game on a new save? I know you could do this is Resident Evil 5, but you can also aim with the right stick in RE5, so that's pretty much a moot point.  
 
Anyway, since the game only allows for one save per storage device, if I had to play through the entire game again, I'd lose my save, and all of my infinite ammo weapons.  
 
Or has no one actually tried doing this? 

I hope not, all I had to do was the easy and hard as well and I started over a new game.  But after about 20 minutes I realized there's no way I could do this again so I never went back.

i saved my original file on a flash drive before doing the other two modes for the achievements. and then i loaded up the original save from the flash drive when i was done with all that and everything was good

Yup, make a copy of your save. If you don't have a spare USB drive I'll sell you my Memory Unit. ;)
I just reached end game tonight, and am starting to do all the Zach events before I start reloading to finish quests.  From what I understand, your inventory is persistent when reloading old chapters, but NOT starting new.  That's like many many other games, so whatever.  Be deliberate and plan ahead.
 
It is very unlikely that reloading chapters will give you a new difficulty achievement, because of how deliberately they designed the limitations.  I can count on one hand the number of games I've seen that let you get away with that.
 
Also, fffuuuuuuuuccckk this game is batshit crazy!  Now I can finish watching the ERs. :)

Reloading chapters makes sense if you beat the game on Hard and are going back for sidequests and don't want to have to slog through the battles to finish... The radio is funny because it seems to have kept track of all the places you've visited for each chapter, so in the earlier episodes I can hardly teleport anywhere.

Good to know, though; I'm going to start over just to read the sidequest dialogues again!