Use Renumber Subaccounts to rewrite the account codes for all children of the current account. For example, you have the following account structure:

You now need to add the “Green” account, but you want it to sort between the Yellow and Blue accounts. Instead of having to manually renumber all the accounts starting with Blue, you can use the Renumber subaccount command on the Colors account. In the pop-up dialog the prefix value should default to “10”, the account code for the colors account. If you set an interval value of “5” and click Renumber you will end up with the following account structure:

Now you have room in the account code numbering space to add the Green account with an account code to force the proper ordering.