@Egbert Nosh 

QBO uses FIFO inventory valuation, which is why you cannot adjust an item's stock value like you recall in Desktop.  The only reason you could adjust the item's stock value in Desktop is that Desktop Pro, Premier, and Enterprise use the average cost valuation method.  If QBO used average cost, there would be an item value adjustment like there is in Desktop.  Desktop Enterprise allows you to choose FIFO, and if you do, you can't adjust an item's stock value then either, so this really isn't a QBO issue.  

There are a few ways to adjust an item's stock value in QBO.  You can zero out an item's stock value by making an inventory quantity adjustment to $0, then re-enter the quantity at the new cost using an Expense (New > Expense) transaction.  The 'Payment account' is the adjustment account.  You may need to set up an OCA and then journal the adjustment to COGS if COGS is your adjustment account.  There are a couple of other ways that use Expense and Credit Card Credit transactions and an adjustment account to raise and lower an item's stock value.