A lot of plans and thinking goes into how to transition from the old to the new. Valuable for the "brief" period of transition but generally worth less once complete. This also doubles the time to deliver. Additionally, many times research and code analysis/review is needed to flesh out what needs to be done. Often times it's faster to make the changes when discovered rather than having to document what needs to be changed then getting the go ahead to change it. ("What did I change?" It's in the code commits and repo! Why do I need to translate it to English? Oh because my boss can't do my job...) This can drastically reduce delivery time. "What if there's a bug?!" What if there is a bug. We'll deal with it.