I did it with my helmet, worked out just fine. In fact I've modified quite a few of my pieces. As long as your knife is sharp and you don't mangle it while cutting it should be fine.
It is entirely possible we missed some things. We had to do a LOT of shuffling/sorting/decision-making when trying to get those in order. There were many times where two files appeared to be the same item in which case we tried to pick the more complete or the "better" (I use that word carefully as I don't want to offend anyone) one. I am sure there are instances where we left something out, thinking it was a duplicate.Ooo that's a good one. Satchmo does great work. Thanks eiros I've made a note of it in the armory. When Art Andrews redid the armory he left out a lot of "duplicate" files. I don't think he realized that some of them were different. Like my helmet is Carter's helmet, but with a bill added on top, so it is a different file than the standard one.