I was playing around with this in BT4 (thanks xX_Spiidey_Xx) and for some reason I have yet come to understand it worked the first time, but refused to change the password any subsequent times.
However this similar set of steps worked every time:
Note: You can also add "panic=5" after the "init=/bin/bash" on the kernel line to have the machine automatically reboot in 5 seconds after it hits a kernel panic
Press ESC at Grub prompt on boot
Select first line, hit e
Select kernel line, hit e
Add "init=/bin/bash" to end of line preceded by a space
At prompt, run "mount -o rw,remount /"
Run "passwd"; enter new password in both following prompts
Run "mount -o ro,remount /"
Hit Ctrl+D to start a kernel panic and hang
Power off machine and reboot as normal
If anyone knows why the original set of commands only worked the first time around I'd be interested. Thanks again xX_Spiidey_Xx