To change nominal account codes after transactions have been posted, you need access to the Corrections module. See Turn on or amend access to use the Corrections module. Once Corrections is available:
Click Financials > Accounts (ribbon menu) or Nominal Ledger > Records > Accounts (classic/module menu).
Load the incorrect nominal account into the Accounts window.
Click Correct.
Confirm the action is set to Change Code.
In the New Code field, enter the new nominal account code.
When satisfied, click the green tick icon.
Your nominal account and all associated records (such as posted transactions) are updated with the new code.
Repeat steps 1 to 7 for each nominal account you want to change.