how to depreciate a duplex, after you've already rented a room

[Reid Isaki]

I have a duplex, i lived in the bottom unit and had a roommate.  Top unit was rented. I calculated 4% of the building was used for the room rental.  I depreciated this value the last two years. He moved out last year and I moved out as well so now I'm renting the entire building.  I had started to use a SL depreciation on the room rental but after only taking 2 years of depreciation i have 25.5 years left.

How do I handle depreciation on the entire building now? even though I claimed 2 years worth on 4% of the building.  

Which forms do I have to fill out and what is the process.

Thanks!