Hi there, @CurtisStevens. I have all the steps you'll need in assigning a class to old accounts in QuickBooks Desktop. 

You can edit your accounts and assign classes to each. Take note that you can only assign classes to income and expense accounts.

Here's how:

1. Go to Lists at the top menu, then select Chart of Accounts.
2. Right-click the account that you want to edit.
3. Select Edit Account.
   
4. Select a class from the drop-down list.
5. Select Save & Close.

These steps are available from our guide on managing accounts in QuickBooks Desktop.

You can also check out this article about importing data in QuickBooks Desktop: Import and export data in QuickBooks Desktop.

Please let me know should you need further assistance. I'm always available here to help.