This option located within all settings > categories & units > change of numbers allows you once the option has been ticked to change the account number of a nominal ledger code within your chart of accounts.

For example, you want to change a code within your chart of accounts. Prior to making the change you will need to have access to this option. Once the box has been ticked you can select the code within your chart of accounts, edit the number and save. A message box will appear asking you to confirm this change. Once the change has been made any historical transactions will also be transferred to the new code.

If you have the additional module Project activated you will also see this option listed.