I've had to deal with this before and I took the same approach as TaxGuyBill.  As usual the Regs are clear as mud and don't address this specific case but you can carve out each "change in use" and follow the appropriate instructions in 1.168(i)-4:

https://www.law.cornell.edu/cfr/text/26/1.168(i)-4

The first conversion (in this case it sounds like that was from business to personal, but we don't know what may have happened before then) is covered in paragraph (c).  The gist of the reg is that it is treated as a disposition for the purposes of calculating depreciation in the year of conversion, however, the reg specifically mentions this regarding recapture:

"Upon the conversion to personal use, no gain, loss, or depreciation recapture under section 1245 or section 1250 is recognized. However, the provisions of section 1245 or section 1250 apply to any disposition of the converted property by the taxpayer at a later date."

So basically the property retains its "depreciation memory" for the rest of its life.

Then we run into another change in use, this time from personal to business, that's covered in paragraph (b).

After babbling on about how to calculate depreciation in the first year, the paragraph finally gets to the point at the end when it says:

"The depreciable basis of the property for the year of change is the lesser of its fair market value or its adjusted depreciable basis (as defined in § 1.168(b)-1T(a)(4)), as applicable, at the time of the conversion to business or income-producing use."

At some point after 168(i)-4 was released, 168(b)-1T became permanent and lost its T.  It can be found here:

https://www.law.cornell.edu/cfr/text/26/1.168(b)-1

"Adjusted depreciable basis is the unadjusted depreciable basis of the property, as defined in § 1.168(b)-1(a)(3), less the adjustments described in section 1016(a)(2) and (3)."

Then keep following the bouncing ball . . . "unadjusted depreciable basis", turns out, is the basis with some adjustments.  But to avoid confusion, these adjustments are referred to as "reductions" and include things like the percentage of personal use, 179, the old investment tax credits, etc.  Basically stuff that's NOT depreciation.

Section 1016 is where depreciation kicks in (turning "unadjusted depreciable basis" into "adjusted depreciable basis":

https://www.law.cornell.edu/uscode/text/26/1016

"Proper adjustment . . . in respect of any period since February 28, 1913, for exhaustion, wear and tear, obsolescence, amortization, and depletion"

And then we get into our old friends "allowed" and "allowable" along with the inclusion of Section 167 by reference to "allowed as deductions in computing taxable income under this subtitle", with "this subtitle" being "Subtitle A. Income Taxes" encompassing sections 1  through 1564 of the IRC.

Coincidentally (or perhaps not!), Section 167 covering depreciation:

https://www.law.cornell.edu/uscode/text/26/167

also uses the terms "exhaustion, wear and tear" and resides in Subtitle A of the IRC.  So unadjusted basis is basis minus reductions minus depreciation.

It's definitely a long journey to get there but I have yet to find anything the clearly outlines a different approach to this.  Doesn't mean it's not out there, just that I haven't found it yet. 🙂

Rick