Is it possible to modify files on sd from PC without unmounting?
Ahh, I should have known, ssh is the solution. Therefore install an ssh server e.g. SSHDroid then you should usually have sftp and ssh to your phone. As command line tools like sed echo and basename are available even renaming is really easy :D.

The really great thing is you even don't need a wifi around, you can make your own with wifi thetering and SSHDroid will still work, though you have to disable the presence of a network in its options first.

You don't need to root the phone.