Why did not PO status change from Approved to Requires Reapproval?

Hi,

I have one PO with status is Approved, information in distribution : Destination type is Inventory, Project is A.

After that, I changed Project A to Project B and save this PO.

But status of PO did not change from Approved to Requires Reapproval.

Another case: PO with status is Approved, information in distribution : Destination type is Expense, Project is A.

After that, I changed Project A to Project B and save this PO.

At this case, status of PO changed from Approved to Requires Reapproval.

So, I did not know the reason why and I want the PO status is changed from Approved to Requires Reapproval.