It's simple. An Iron Chest can be crafted into a Silver one, so I believe that there should be an upgrade for that. As in my pack I have it where chests get upgraded by storage size, this makes a great problem as players have to break the Iron Chest to be able to upgrade it.

I too would like to request an iron to silver chest upgrade. In my modpack, players have to upgrade Copper to Iron to Gold to Diamond. I would like silver in there but no one wants to break a chest to upgrade iron to silver...thus everyone skips the silver chest during the upgrade process.