Hello Sidhir,
I did your scenario in my test enviroment and I advice you use the date of deprecioation start in the valuation area that you have to change the usefull life. Ex, If you change the usefull live of 12 to 15 year and it must have began in 01.01.2012 you have to use this date in the depreciation start of the valuarion area.
The adjustement of the value of the asset will be done in the current year. If necessary, the SAP will post the value adjutment in the next depreciation run (AFAB).
Best Regards
Cassio B. Goulart