I listen to every DHH interview I can but his view on a rewrite is different than others that do rewrites.  There is no &quot;second system&quot; per say in his world.  He leaves the old system up for people that still think it works just fine.  Instead of messing up the old versions with upgrades, they leave them up and if a customer wants to go to new version that is &quot;rewritten&quot; they can.