There is a simple solution if you can install and use puttygen tool. Below are the steps. You should have the passphrase of the private key.
step 1: Download latest puttygen and open puttygen
step 2: Load your existing private key file, see below image
step 3: Enter passphrase for key if asked and hit ok
step 4: as shown in the below image select "conversion" menu tab and select "Export OpenSSH key"
Save new private key file at preferred location and use accordingly.