you have some options
1) you prepare form 3115 to correct for the depreciation you should have taken but didn't
2) hire a tax pro to do this. the 3115 can be tricky
correction must be made. the tax laws say you recapture the larger of the depreciation you actually took or the amount you would have taken if you used the correct method and life. so if you don't corret you'll be taken on the 19 years of depreciation you didn't take.
