Your banking information cannot be changed after your tax return has been filed and accepted.
Banking information does not go from TT to the IRS until you file your 2020 tax return and it is accepted. You enter/change banking information in the FILE section before you file your 2020 return. Changing it on the TurboTax site does not do any good until you file your return. If they manage to send out stimulus checks before you can change your information, we hope the IRS provides a way for you to change it with the IRS. If your check goes to a wrong or closed account, it will be sent back to the IRS and then the IRS will mail a check to the address on your tax return.
The IRS only uses the bank account from your most recent tax return if you received a tax refund and selected direct deposit.
The only way to change your bank account information with the IRS is to file a 2020 tax return and enter your current bank routing and account number in the File section of the program. That will be sent with your federal tax return when filed.
If there is another stimulus and the IRS sends the payment to an old account, the bank will reject the payment and send the payment back to the IRS. The IRS will then send the payment as a check to the address on the tax return.