well, some code can be highly interdependent, but it sure seems unlikely to me that HDs job logic would relate to anything else. i would think it would be trivial, but you never know, without seeing the code. the practice planning stuff, seble says he has to rewrite the entire thing to touch anything - im guessing old admin wrote it in a language seble is not comfortable coding in. however, seble already tweaked the job logic significantly, i have to imagine it is something he is comfortable coding in, and would be very easy.
there's always more than coding though, there should be formal QA and release processes, that make development costs for very small tweaks significantly higher than just the development cost. not sure HD uses any processes like that. maybe seble just tests it himself.