I posted quite a few AR Invoice Items to the wrong GL Account, how can I update the GL account on the Journal Entry?

GL Accounts can't be updated for the original Journal Entry, however, you can make a formal adjustment so the CR GL Account  on the AR Item is now correct. 

1. Navigate to the Adjust tab of the Invoice Header record;
2. Select Tentative Adjustment reason as 'Transfer: Different GL'. 
3. Specify the GL Account and the Tent. Adjustment date. 
4. Press Update. 

This will cancel the original (incorrect AR Items) and create new AR Items with the correct CR GL Account.