If it is two separate units, you should report it as two separate units.
For the one that was your personal unit, you enter it as a new rental property, and for the "asset" for depreciation, you enter January 2018 as the "placed in service date". You will enter 50% of the total Basis (plus or minus any Basis adjustments on the that unit) and say it was used 100% for rental.
For the 'old' rental side, you also report would report 50% of
the expenses and 50% of the Basis (and 100% of the "prior depreciation") for the asset for depreciation, using the original "placed in service date". You
also say it was 100% rental.
So the end result is that you have two rental properties, each claiming 50% of the expenses and 50% of the Basis for depreciation, and both are being 100% rental use.
Does that make sense?
