Note: As this merger completes, and the broker migrates data, the supported import methods will change. Those that were previously able to use BrokerConnect will lose access to this import method.
Since you will receive two separate 1099-Bs - pre-merger 1099-B from TDA, and post-merger 1099-B from Schwab - we recommend that you have two separate account tabs in TradeLog: one for the pre-merger TD Ameritrade data, and one with the post-merger Charles Schwab data. This will make the 1099-B reconciliation easier.
You'll want to import up to the merger date in the TDA account.
Then, press the "Open" button to view/verify your open positions in the TDA account, and click the "Transfer Open Positions" button to transfer all the TDA open positions to a new account tab for Charles Schwab. See: Transferring Open Positions.
Then go to the Schwab account tab and you can continue to import your transactions from the post-merger date onward, by following these import instructions: Importing from a CSV File - Charles Schwab
If you are not yet able to download your new trade history from Charles Schwab, then you should be able to import via a CSV file from TDA: Importing from a CSV File - TD Ameritrade