Can I go back today and declare personal use?
Are you asking this because you've already e-filed your 2020 tax return, and it's been accepted by the IRS? If yes, then you'd be better off converting it to personal use on 1/1/2021 when you file the 2021 tax return next year. That would stop depreciation on 1/15/2021 or thereabouts, since the mid-month convention is use on residential rental real estate. Then you'd be including the SCH E in your 2021 tax return for the sole and only purpose of the conversion. Doesn't change anything else covered previously, about the 2021 tax year, as you'd have no rental income or expenses to report on the SCH E for 2021 at all.
Generally, you only amend a tax return if doing so will make a difference in your tax liability. Amending the 2020 return merely to convert the property to personal use on Dec 31 will not change your 2020 tax liability by a single penny. So not worth bothering with really.
