Trial Balance report is a standard report that is designed to show the balance of the GL accounts at a specified date/period that the user have selected.
If you wish to view the initial balance of the GL accounts, you have these options below.
1. Set the As of (date) to the date that the initial data migration was completed.
Part of the Data Migration process is to load the account balances into the system from your legacy system.
2. You can navigate to List > Accounting > Accounts or Setup> Accounting > Chart of Accounts to see the list of GL accounts.
Then click the specific account name to go to the register page which will show you all the transactions posted to against that account event the initial amount.
3. Alternatively, you can use the General Ledger report and set the From/To periods accordingly.
I hope this helps.
Let me know if you need further assistance.