When is the account determination carried out? In the moment you do a posting. So a change of the G/L account will become effective immediately with the first movement after the change request got transported in the system.
Depending on the transaction it can have unwanted effects. one of them is explained with screenshots in KBA 2008455 - Change of account determination when stock is not zero
Others are effects may be seen if you cancel a movement after you changed the account, as it hits the new account instead the old account from the original posting.