There are two steps to changing the payroll bank account in QuickBooks online. One step to actually change the bank account routing and account number, and then another step which changes the account payroll defaults to deducting from.

I just dealt with this same issue myself, I changed which account it should be deducting from but funds were drawn from the old account. why can't this be one step quickbooks? this is much harder than it needs to be.

Under the great icon, go to payroll settings. next click "direct deposit", there you change the account and routing number to the bank account you would like funds withdrawn from. 

Next under payroll settings go to "preferences" and then "accounting preferences" at the top there will be a drop down menu, this changes the account that payroll is entered into.