I would like to add two additional questions to the thread:

1. When consistent tax treatment has been established, is form 3115 the only option or amending is also accepted solution? In this case, amending two years is not too bad technically.
2. Suppose the client also "forget" to carryover the rental loss in 2019 from loss from 2018 to 2019 (or something else), what is the solution in this scenario?