I have lots of feature modules in my site. I want to
import it back to database for upgrade purposes. Any idea how can I do that?
I used this patch from here - http://drupal.org/node/1014522#comment-5719202
After you patched it you can then use
drush features-import-all --yes or
drush fia -y to import features to database.
There is a patch in the works for this, Provide way to Import views/true exportables back to the database. I have given it a quick test and upgrading seemed to work out fine, however, that particular project ended up with building a new site from scratch anyway, so I haven't used it a lot.
You can use the features tools module to import the features from code back to the database.
Note : This will really help the performance of the site.