increased useful life

Hi,

We made some repair costs to our building, and we decided to capitalize the repair costs to value of the building. Also we decided that repair works incresed/prolonged useful life of this building. We want that NBV and capitalised repair costs would depreciate during the remaining + prolonged periods, i.e.

Initialy useful life was 30 years, after 20 years when we made repair it was 10 years remaining, and we decided that repair prolongs useful life additional 5 years, so in total 15 years.

How to adjust remaining useful life in order to claculate depraciation for the rest period plus additional period smoothly? Because system recalculates depreciation of previous periods and posts positive values and than negative, but we want calculate smoothly for the whole rest depreciation period with negative depr. values.

Please help!