@PhoebeRobertsTruly appreciate your answer, suppose:

1. there was missed depreciation in both 2018 and 2019
2. there was no rental loss carryover from 2018 to 2019
3. we know that there were other errors (overstated deduction) that may or may not change rental income

My questions are:

1. Are we permitted by the law to correct depreciation via form 3115 ignoring the other errors that we know?
2. Suppose we need to amend the returns for the other errors, can we correct the depreciation at the same time or we have to use form 3115?
3. Similarly, can we just carry forward the rental loss to 2020 as filed originally ignoring the other errors that we know?
4. Can we choose not to file amended return for 2018 and 2019 but to use the correct rental loss amounts in 2020 as if the amended return correcting the other errors were filed?
5. Can we mail the amended returns correcting the other errors to clients and assume they were filed and proceed with the assumption?

In the other words, what are the correct options for us to take in above scenario?

PS: I believe your previous answer:

The amount carried forward into 2020 is required to be the "as if prior returns had been prepared correctly" amount, whether the client chooses to amend 2019 or not.

is correct, but can you provide the reference document for me to study more?