The moment you start changing posting groups your reconciliation has gone, you would need to ensure you knew the position of every item part processed at the change, manually journal the values across and then continue. Whilst you can change these it does not mean you should, and if you do, as in this case, there is a significant amount of manual work to reconcile that will need undertaking.
yes that has exactly the same impact. In essence it hits one account and then as it moves it would take it out of that account, however as you have changed accounts it will take it out of an account it was never in instantly creating mis-matches. Not something you should do without very careful planning.