It’s actually a very easy change to make. It will require that you restart the SQL Services on all your services though.
Use the Services Admin Tool (or control panel applet on Windows 2000) to change the password for the actual services on all the servers. Do NOT restart the services yet.
Once all the passwords have been reconfigured, reset the password on the domain. If you are in a multi-site environment have the domain admin force replication.
Restart all the SQL Servers.