Lisa Price at I.B.I.S. had a nice tip today that I had never thought of. When companies are transitioning to Microsoft Dynamics GP, they often make changes to their chart of accounts to improve reporting or data entry. Lisa uses the account alias functionality to hold the old account number when possible. This eases the transition between systems because it allows users to lookup their old account in the alias and find the new account.
If there's not a one to one relationship, it's not perfect but if you're splitting or combining accounts in the chart, you probably don't want much of a link back to the old account anyway.