the problems aren't usually technical as much as political or 'business' generally (though sometimes technical stuff presents challenges, certainly). There's a

the problems aren't usually technical as much as political or 'business' generally (though sometimes technical stuff presents challenges, certainly). There's almost always an assumption, I've found, when coming in to existing projects, that they "really" work a certain way and maybe there's just "a bug here" or "some small fix needed over there". Sometimes that's the case, but more often than not, even after those things are 'fixed', nothing actually does what the people in charge thought it was doing. They've sometimes fundamentally misunderstood what they'd required/specced out with a previous team, or, in some cases, the previous team really didn't know how much they didn't know, or in a couple case, they were simply lying for months on end, cashed checks, then left. "rewriting a system you don't understand is pretty hard" getting a team to successfully spec out what they need is pretty hard, imo, regardless of whether it's greenfield, rebuild, or clarifying expected behaviour in a current system. The big challenge I've come across with "incremental changes to existing systems" happens when it really doesn't do (and never did) what people thought was happening. "No, this isn't calculating daily compound interest - it's calculating monthly." "No, the reports aren't able to be regenerated from previous points in time because historical information was never stored anywhere - you simply now don't have 4 years of data you thought you had - the previous reports you were told were being regenerated were just being pulled from a cache." Fixing problems like above may mean battling through lots of bad code and many many contentious discovery/planning/testing meetings, and... the result is now someone has a 'functioning' system running on codeigniter 2 with hard dependencies on php 5.2. Years ago I faced a situation with a Rails app which "just needed a few modules finished and a few tweaks". It was done in Rails 1 and had insane amounts of runtime monkey patching. 3 different Rails shops I went to indicated it was not worth the time/effort to try to 'upgrade' what was there. They'd rebuild greenfield and salvage what bits they could from the previous project. Many of my experiences have been with smaller organizations (less than 50 people - sometimes less than 10), and there's rarely a full team of competent IT people who can review/vet/verify. "Never rebuild from scratch" - it's not bad advice, but it has a lot of assumptions that aren't always true.