I changed banks between the time the first stimulus was issued and the time the second check was issued. How do I reflect the amount of stimulus due me on the 2020 tax return?

If your stimulus was sent to a closed account, the IRS will not issue a replacement.   You'll have to claim it as the Recovery Rebate credit on your 2020 tax return.    

In TurboTax Online, to claim the Recovery Rebate credit please do the following:

1. Sign into your account and continue from where you left off
2. Click on Federal in the left-hand column, then on Federal Review on the top of the screen
3. On the next page titled Let's make sure you got the right stimulus amount, click on Continue
4. Follow the interview
5. TurboTax will determine whether you are entitled to any additional stimulus
6. Any stimulus amount remaining due to you will show as a credit on line 30 of your form 1040.

It is my understanding that the latest stimulus check is based on 2019 adjusted income but it appears Turbotax is basing the calculations of the stimulus check due me based on 2020 adjusted income.  Am I correct?

You are correct.  The Recovery Rebate Credit on your 2020 tax return is based on information in that tax return.  Any round one or two stimulus payments were advances on your 2020 Recovery Rebate Credit.  However, since at that time you had not filed a 2020 return, those payments were based on prior year (2019/2018) return information. 

Here is a link with more information on this topic.