gr/ir posting rule to be modified.

In our company till now in SAP MM single GR/IR in (WRX) is getting posted into SAP FI since 2007. Now the management wants to bifurcate the GR/IR account according to different valuation classes.

Can we change the posting rule in the middle of the financial year ?

What will happen to previous pending purchase orders and any implications on cancelling MIRO and MIGO?

regards

ravi