Not sure if it's enough. But:
Edit /etc/passwd and /etc/shadow as root and replace
jim with mike in the line where jim appears.
[code:1]
jim:x:503:100:Big Jimmy's real name:/home/jim:/bin/bash
# Change to
mike:x:503:100:Big Mikey's new name:/home/mike:/bin/bash
[/code:1]
then rename the home-dir.
[code:1]
/> cd /home
/home/> mv jim mike
[/code:1]
This should work. But I will not guarantee anything :lol:. Consider triing it with some test-user first ;-)
Expect the worst, it's the least you can do.