Hi,
As far as I Know If you want to change the GL account for accounting entry which got posted during settlement, only way to do it to reverse the settlement and re do it again.
Before that you have to update the desired GL in OBYC->GBB->AUA and AUF. because for production order settlement system pick up GL from here...
To restrict the GL at order type level, i think you should have order type at plant level and map plants as per valuation grouping code.
Reg
karthik