You didn't indicate what cartidge you are trying to change to, only 8mm caliber. If it is 8mm-06, no problem, in fact that's the only realistic way to get 8mm-06 brass. If you are talking about the much shorter 8x57 (8mm Mauser), then you will be moving the "new" neck to what was the case body, and because you reduced the size considerably in this new neck area, it is almost a necessity that you turn the necks, otherwise they will be too thick, causing a lot of resistance when the bullet tries to make it's exit. This will result in big-time preasure! As to the forming process, if you use Imperial Sizing Wax, you should have no problem. I once made a bunch of .300 Savage brass out of '06 (.300 Sav brass wasn't available for a while). This was a bigger alteration of the brass than going to 8x57, and I had zero failures. But you must turn the necks, or you could create a real problem.