I think those were drafts. The Dilbert strips are created using some computer illustration software (Adams even has his own private font that he uses for letter

I think those were drafts. The Dilbert strips are created using some computer illustration software (Adams even has his own private font that he uses for lettering the strip). So changing the punchlines to something funny wouldn't have taken too much effort. Apart from coming up with the funny punchline, that is.