I originally generated my public/private key pair with the putty client and
converted to putty .ppk format with puttygen. I no longer have the orginal id_rsa key.
I'm attempting to convert the putty format secret key back to openssh format.

So I went ahead and saved my putty private key in OpenSSH format using puttygen.
I created a .ssh/id_rsa file on my home Linux machine from which I am connecting, pasted
in the exported key, 'chmod 600' and then tried to connect to the host running sshd
with port forwarding enabled as follows:

$ssh -2 -l <username> -L 888:127.0.0.1:443 x.x.x.x

I get the legal banner from x.x.x.x and then I'm prompted for my passphrase
as normal. However it won't accept my passphrase even though I know it's
correct. Has the export in puttygen stripped the passphrase do you
think? Not sure.

I've tried 'ssh-keygen -p' on the client to force a passphrase change but
again it doesn't like the old passphrase I give it when prompted. It seems
from Google that converting keys from one format to another is not
really recommended. Anyone have any clue? My only other option to re-gen a new
set of keys for my home machine to ssh to this server.

Thanks!

This is actually a better idea, even from security point of view.
If you still need just one key-pair, can you export your key in puttegen with no password and then set a password using openssh?