They can be changed on active inventory, but currently only one item at a time. There currently is no 'mass change costing method' in Adagio Inventory.

Should you do this, FIFO costing lots would get merged together into one. I shouldn't advise (or reject) doing this change, as it should reflect how you do business. I likely wouldn't change solely for making reports on average cost easier to do - there should be a business case for making changes.