QB solution to the problem (one by one):

Here's how:

1. Go to Lists and select Item List.
2. Double-click on the item you want to update.
3. Open the item, change the account (to dummy or different account).
4. Click Yes if you'd like to update existing transactions for this item.
5. Open the item again change the account to the correct account.
6. Save then Yes to update all existing transactions.

Unfortunately that means 000 hrs fixing 00,000 items.  No real solution.  Easier to switch accounting software providers.