Is the backupfile of m0n0wall including the old password?

And this is why I like a plain text config file! In the future, you can also reset the admin password to default (small) from the console, but a copy and paste in the config file is a good way as well. Lastly, if you have multiple users and they have access to the general setup page, they can change the admin password.

But changing things with the config file is a supported method of doing things if you are careful. I use it all the time to do things like change the IP address range from 192.168.1.x to 172.16.10.x for example with a global search and replace on the entire file. That gets all your NAT, static DHCP assignments, the IP range... Everything but the statis IP devices on your network.