Can the point at which the requisiton encumbrance is created be amended?

R12.1.3

When requisitions are raised, the current encumbrance setting is to encumber them when they are submitted.  Our client has asked if it would be possible to change this so that the requisition is only encumbered when it is approved.  Under the present setup, if requisitions are rejected (and never approved) they are still showing as an encumbrance until the requester cancels them.  This is causing operational difficulties, as the requesters don't know they should cancel the requisitions if they are rejected.  It would be more helpful if the encumbrance only occured when the requisition was approved. 

However, if this set up is changed, what will be the effect upon existing requisitions and the encumbrances that have already been created?