You can change the comment for RSA1 keys using
from the ssh-keygen manpage:
-c Requests changing the comment in the private and public
key files. This operation is only supported for RSA1 keys. The
program will prompt for the file containing the private keys, for the
passphrase if the key has one, and for the new comment.
So, to change the comment of a key located at
~/.ssh/some_key, use the following command:
ssh-keygen -c -f ~/.ssh/some_key -C "my new comment"
-f option is followed by the key you want to change, and
-C is followed by the new comment.