@LouiseG 

"At this moment, the option to change inventory adjustment accounts in batches is unavailable in QuickBooks Online (QBO)."

Yes, you can make batch inventory adjustments in QBO:

Go to Settings > Products and services. Click the funnel (filter) and under 'Type', check the Inventory box. Then, click the box to the left of 'Name' to select all items. When you do that, you will see a 'Batch actions' drop-down appear on the right. Click 'Adjust quantity' and you can adjust all inventory items at once.