Changing the IP address of a firewall, or for that matter any device, all depends on what access you have to the network.
For instance you may have a VPN which bypasses the ASA and allows you access to the management network, or a console server which can be dialed up.
If you are relient on accessing the firewall via it's public IP then I would suggest either being on-site, or having someone trustworthy there to to the work on your behalf.
