What would happen if a Feruchemist fills, for example, a tin metalmind then mixes it to make a pewter metalmind? Does the stored attribute change? Is the Investiture gone when you melt the metal? What if he just makes it into a tin metalmind again?
If you make it impure, you'll keep the investiture, but won't be able to get it out. If you make it back into the same thing, you'll be fine, and can access it normally. If you try to fill it, after changing the composition to make another viable metal, it will act a little like a computer hard drive with corrupted sectors. Some of it will work for the new investiture, but you won't be able to fill it nearly as full. (Depending on how full it was before you melted down.)
This holds for basic uses of the metallurgic arts. Once you start playing with some of the more advanced parts of the magic, you can achieve different results, which are currently RAFO.
Similarly, if you were to soulcast a metal would it have similar effects of corrupting the investiture and making it inaccessible? Like if you turned a steel metalmind into pewter.
I've stayed away from soulcasting and forging in these types of discussions, as I feel my answers will dig too deeply and prompt more questions that, eventually, will lead to lots of RAFO type questions. I don't really want to go there--but I will say this. Changing invested objects with other magics is hard, and often requires such a force of investiture yourself, that it becomes very power-inefficient. Just like we can technically turn lead into gold right now--by spending way more money than the gold is worth.
So you could, for example, use electrolysis to dissolve a metalmind in water, then reverse the reaction later to get the investiture?
OR, better question, if you store investiture in one allotrope of iron, can your retrieve it off you change to a different allotrope?
I see no reason why these wouldn't work.
So would forging with the blood of a radiant(kaladin, dalinar,etc) work on a shard blade from a fallen radiant to say change who they had bonded, or how the bond was broken (to say death instead of giving up on the oath)?