Basically, any player that plays in a position where is lighter weight than the ideal average will have an advantage over any other player at that position.
Some common switches that I like to use for this reason:
All LBs at WLB, all DBs at CB - The difference between WLB and MLB is 8 pounds. That's only 1 point of SP, but noticeable in terms of AC. Same with DBs. You wouldn't think that the 12-13# difference between CB and SS would matter, but it does...oh it does.
WRs at RB - The classic. The acceleration difference between 217# and 197# is very noticeable. 197# gets to the line faster than 217# every time.
FBs at RB - Same with above. There is a huge difference between 243# at FB and 217# at RB. Everyone associates this with the gain of 3-4 SP, which helps, but it's also the acceleration of the player that allows him to get past LBs on wheel routes and not get blown up in blocking. I have had 77 SP 217# FBs that are absolute monsters in FB passing games.
As I said in the other thread, AC to my knowledge was never normalized for weight. That means that you can't assume the number on the card is the player's AC. How SP _used_ to work, and I'm sure how AC _still_ works, is that the number listed is for the weight class.
For example, it used to be that all players could reach 100 SP. (This was way, way, way back in the 3.x - 3.8 days [I don't remember the exact versions.]) But then your DT would chase down a WR 50 yards downfield (hilariously true).
So JDB introduced weight into the equation, but didn't change the numbers. That meant a 100 SP DT was the fastest DT you could have at 300 pounds. Now, the numbers don't go up to 100, but the idea is the same. The 89 AC WLB is the fastest accelerating WLB you can get, but he's still slower at accelerating as an 89 AC RB.