Why did Microsoft disable this ability in the first place?  Please read their statement below. 

If you choose to use GP PowerPack to override this control, it may have other consequences in the system.  You assume all responsibility for the use of this Tweak.

In order to facilitate the matching of the inventory value in the Inventory module to the inventory value in the General Ledger module, a change was made beginning in Microsoft Dynamics GP 9.0 Service Pack 3 and Microsoft Dynamics GP 10.0 Service Pack 2. You can no longer edit the inventory account number in the Purchase Order Processing module. This change applies to summary level changes made in the Purchasing Distribution Entry window for the PURCH account type. This change also enables the Historical Inventory Trial Balance (HITB) report to balance.

If this activity were allowed, it could cause a situation where the inventory value in the Inventory module would not match the inventory value in the General Ledger module.

More Info Here