Can You Change Acct. for Inv. Line When it is From a Matched Project-Related PO Line?

I entered an AP invoice line, matched the line to a PO line. The PO line was originally created for a Project ABC, Task1.

In the invoice line distribution, I proceeded to updated the related project and task combination, say to Project XYZ, Task2. The acctng. flexfield is not updating accordingly. 

Is this standard functionality, or is there something wrong with how we customized our AP account generator.

Your input is highly appreciated.

Thanks, Rosalie