Ssh – How to reuse ssh private key

UPDATE: For an actual answer see zero_r's answer below

This isn't an answer, but a workaround. Use screen.

When you first log in, run screen. You get another shell, run commands in that. If you're disconnected, the screen process keeps the terminal alive so that your shell and the processes it is running don't fall over. When you reconnect, run 'screen -r' to resume.

There's a bunch more to configuring and using screen, but the above should workaround your problem.

To change the passphrase on your default key:

$ ssh-keygen -p

If you need to specify a key, pass the -f option:

$ ssh-keygen -p -f ~/.ssh/id_dsa

then provide your old and new passphrase (twice) at the prompts. (Use ~/.ssh/id_rsa if you have an RSA key.)

More details from man ssh-keygen:

[...]
SYNOPSIS
    ssh-keygen [-q] [-b bits] -t type [-N new_passphrase] [-C comment]
               [-f output_keyfile]
    ssh-keygen -p [-P old_passphrase] [-N new_passphrase] [-f keyfile]
[...]
     -f filename
             Specifies the filename of the key file.
[...]
     -N new_passphrase
             Provides the new passphrase.

     -P passphrase
             Provides the (old) passphrase.

     -p      Requests changing the passphrase of a private key file instead of
             creating a new private key.  The program will prompt for the file
             containing the private key, for the old passphrase, and twice for
             the new passphrase.
[...]