I doubt many of the cases would split during fire forming, people fire form cases all the time. Occasionally you lose one, but that is the exception rather than the norm.
The biggest question I would have is what would it be used for? Informal range time? Hunting? I know, "why do it?" isn't the question, the question is "why not?" The OP will have many opportunities to ask himself why, let's not try to discourage him. But it sounds like a fun exercise, nonetheless. About 35 years ago, a friend asked why I didn't turn my 22 Hornet into a K-Hornet, and I asked him why I should. He eventually bought the rifle from me and did the improving, learned a lot and had a good time doing it. This is about the same thing, improve the 7.62x39 and gain about 200 fps but have something unusual to show off. I say do it!