I usually carry the battlefield keys with me even though that does not seem necessary. Then the stranges thing happened when I entered act 2. I found the first key and then decided that it was time to drop the others. I jumped to the battle field and came back again after some time. I see my keys which I forgot I had thrown there and went to pick them up! And then suddenly I had access to the level 2 and 3!!!! Is that a good bug of some sort??? Naturally I jumped back to the battlefield to check whether they had infact been opened and yes they had!!!
if your current save game has them when you upgrade, they're still there, but now they all access battlefield level 3. even the new ones you pick up do the same.
