Once you defined the valuation area is a plant then it is not possible to switch the valuation level to Company code.
Your decision also affects:
- Maintenance of material master records (for example, accounting data)
- The G/L accounts in which the material stocks are entered
Please refer the following links,
https://help.sap.com/saphelp_46c/helpdata/en/75/e364b20437d1118b3f0060b03ca329/frameset.htm
http://help.sap.com/saphelp_46c/helpdata/en/f0/0c1ab0b44e11d29c7a0000e8a51c6e/content.htm