Need to adjust the unit cost of a item

Our inventory items are valuated via FIFO. I have some items that ended up getting a $0 unit cost, but those items are layer #2 in our cost layers and I’m unsure about what I need to do in order to change that unit cost. For items where the $0 was the first layer I could just issue out the item and do a receipt with the new unit cost. Yes, I am aware that it would move that cost layer down to the bottom of the list which is okay in this scenario.

If I need to change the second layer do I need to do 2 lines in my issue, remove enough inventory to relieve the first layer and then do a second line that issues out the 2nd layer, and on the receipt receive enough that mimics the first layer at the original unit cost and then a second line that has the adjusted unit cost?

I have read that it is recommended to do adjustments to change unit costs but I find that those changes are not reflected in the cost layers GI.