Not sure what you exactly mean by "added a new payout preference". There is changing (or adding) a payout method and there is changing (or adding) a payout routing rule. Steps you should have taken (if not already):
1. Add a payout method with the new account details.
2. Set the new method to default.
3. Remove the old account.
4. Alternatively, instead of removing the old account, you can set up a new payout routing rule and set the new account to 100%.
As for the payouts that will be affected by your changes, this is what it actually says when adding a new routing rule: "Note: updates to routing rules will only affect bookings made after today."