Clear and start over if the change would alter what was reported in 2020. If you are dropping a dependent from 2019 to 2020, there would be no need to clear and start over. Simply update 2020.
If 2020 will be impacted significantly, then be sure to file the amended 2019 return before submitting 2020.
The only way to amend 2019 is to use the CD/Download. The TurboTax Online program has been updated for the 2020 tax rules already. For instructions, see: How do I amend a 2019 return in TurboTax?
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