You can just delete or mark as inactive your TD Ameritrade accounts in Mint and then add your Charles Schwab account (which, if you'll notice, combines all TDA accounts into one CS account). The transaction history in Mint for my TDA accounts were already gone. When I added my CS account, it pulled in the transaction history.
To edit your Mint accounts (e.g., mark as inactive or delete accounts): https://mint.intuit.com/support/en-us/help-article/bank-accounts/mark-account-inactive/L5koMHE77_US_...
To add a Mint account:
1. On web app: In the menu bar at the top, select the "+ Add Accounts" item (3rd from the right (between "Leave Feedback" and "Notifications").
2. On mobile app: On the top right, tap the profile pic icon. Tap the "Account" option (2nd from top below your full name & e-mail address).
