what would be the best way to add serial number tracking to items already in inventory?
Normally, the best way is to use the Item Reclass. Journal as it will take your costs and flow them from the old S/N to the new S/N. The only problem is that without modification (in 3.70 anyways), this Journal doesn’t like to change from Blank to non-Blank. (Which is what I think you want to do here)