Change Limit PO line item account assignment

SRM 4.0 SP12

Extended Classic Scenario (R3 4.6C Backend)

I've read in a few forum posts that it is standard behavior to not allow account assignment changes on limit item POs. Can someone explain why the account assignment shouldn't be changed if no follow on documents have been created? Is there an SAP note which explains why changing the account assignment on a limit PO is a bad business process?

Thanks in advance!