"Dig into how it works" may take longer than rewriting or it may not. And the rewrite may (and often does) run into unexpected difficulties too. I think it's a

"Dig into how it works" may take longer than rewriting or it may not. And the rewrite may (and often does) run into unexpected difficulties too. I think it's a mistake to make the "fix the old" or "rewrite from scratch" decision up front from a position of high uncertainty about the old system. I'd go with a strategy of starting a timeboxed exploration of the old system with the objective of understanding it better, hopefully well enough to have a reasonable idea of what it will take to fix it. Then make the decision from a better informed position. Yes sometimes the decision may still be to rewrite because you now know that the old system will be excessively hard to change but in many cases you may find out that the old system wasn't as bad as expected.