Change a contract after a PO has been created

Hi,

I would like to know if it is possible to change a contract after a PO has been created with reference to that contract.

Is there a condition for the change? I mean, I would like know the stage at which the contract can be changed:

as soon as the PO is created

GR has been done for the PO

PO has been invoiced.

Regards,

Lakshmi