Update the ssh key in authorized_keys file on remote server, adding a new preferred key and removing a deprecated one

ssh-update-key

#!/bin/bash

# The use case is that I have an 1024-bit RSA key which I have used for many years to access many remote servers.
# I want to stop using it (don't load it into my ssh-agent), and instead use my new ED25519 key.  
# I need a convenient way to update the authorized_keys file on a remote server.

# This solution is tested and working on Ubuntu 20.04 host
# It works on many distributions/versions of remote servers, including Ubuntu 16.04/18.04/22.04 and CentOS 7.9.

# Instructions:
# Set the two variables below
# Invoke using: ssh-update-key account@someserver.com

### Configuration

# Create ~/.ssh/deprecated directory and move your deprecated key (private and pub) into there.  Then set this variable.
deprecated_key=id_rsa

# Set this variable to your preferred key.  Should be located in ~/.ssh
preferred_key=id_ed25519


### Script 

# create symlink, so that ssh-agent will find and load the deprecated key
# (this happens automatically for me on Ubuntu 20.04)
(cd ~/.ssh && 
ln -s deprecated/$deprecated_key . && 
ln -s deprecated/$deprecated_key.pub . 
)

# copy the preferred key to the remote server
ssh-copy-id -i ~/.ssh/$preferred_key "$@"

# now remove symlinks (only if they are symlinks - don't want an accident)
(cd ~/.ssh &&
 [ -L $deprecated_key ] && rm $deprecated_key &&
 [ -L $deprecated_key.pub ] && rm $deprecated_key.pub
)
# ssh-agent does not reliably remove keys if the files are removed.
# kill it to make sure.  For me on Ubuntu 20.04 it respawns automatically.
killall ssh-agent

# Now remove deprecated key from remote authorized_keys file
# the awk regex escapes / characters which would otherwise mess up sed.
ssh "$@" -o PasswordAuthentication=no "sed -i.bak '/$(awk '{gsub(/\//, "\\/"); print $2}' ~/.ssh/deprecated/$deprecated_key.pub)/d' ~/.ssh/authorized_keys"