Flat tappet cams and some hydraulic rollers are cast iron/steel (I don't know the specific alloy) and thus have same hardness throughout. You can regrind to your heart's content and not have to worry about hardness. Bigger Roller cams are made from a steel alloy (something like 4140?) that is then surface hardened partway into the metal. On these, you need to be careful because if you go too far you can get through the hard layer. I recently had a quote from a grinder and the solid roller I wanted to match my heads was only a bit over .600" lift and his available cores were for bigger lift cams and would go through the hard layer to make something so small, so I couldn't use him.