Can we change the Valuation Group code once postings are made

HI Experts,

1.My Client is having requirement of revamping the valuation group codes.In details:

Client is having more than 10 comp codes. for ex:one company code with 5 plants.In each plant he procures materials for 3 departments Manufacturing ,Admin and sales.

He wants to differentiate these procurements by creating 3 valuation Grouping codes i.e VGC-Mfg,VGC-Admin and VGC Sales.

Is it possible and is it correct way?As per SAP we cannot assign One valuation area to more than one VGC.

2.Also Any idea on " Automatic G/L account determination should be based on Cost Center/Profit Center during Goods Movements"

Points will be rewarded if the solutions are accepted by Client.

Regards,

Chris.