@TaxGuyBill 

I don't agree or disagree with that, I just think it's an interesting question and I can't find any example of it being definitely resolved.  It might depend on the definition of "disposition" in  Treas Reg §1.168(i)-4(c).  I came across that reference in a 2017 discussion from "TaxProTalk," here:

https://www.taxprotalk.com/forums/viewtopic.php?f=10&t=9359

Where the moderator Frankly may have a point.  

If changing it from personal to business is a "disposition" for these purposes, then the previous depreciation would have to be recaptured and not subtracted from basis.  I have seen stranger results.  But maybe this is more like a vacation home, where some years it is rented 14 days and others 15 or more.  What do you do there (especially if the FMV is declining)?

I hope to never get this question, but my instinct would be to continue the earlier depreciation for the original building, then depreciate the improvements with a 2019 start date.  I would be open to suggestions, though, about why that is wrong.