It's too late for this project, but if you had used Pack-and-Go to copy the original assembly to a new folder, you could have specified new files names and if you click "include drawings" then the drawing references would have been automatically updated.
You will get a dialog box where you can browse to the new file. You can also specify all the views where this model appears so they replaced in one step.
Gnanesh, you can I believe use these steps.
1- Create a copy of your drawing (CTRL C/CTRL V in Windows Explorer)
2- In SolidWorks, use File > Open and select this copy (do NOT open)
3- Click the "References" button
4- Double click the current reference assembly which opens a browser for you to choose the new reference
5- Click OK and the text goes green (to show a change)
6- OK again and open the drawing- you may get a bunch of "dangling" references (dimensions, notes, balloons etc) but the views should update to the new reference and save a fair amount of re-working. If the new assembly is largely based on the old assembly though then hopefully there is not too much to tidy up.