tangor - 6:17 am on Jan 12, 2012 (gmt 0)
I'm in the "just do it" group. But will say that before any roll-out dev testing/checks have been done and there are near zero internal link transfers for content. That said... I've done that with general success (not 100%).
But killing a site? One would have to qualify what is a "kill".
If this is a money site making money, move with due caution. Keep in mind that sites which have page dates back to the 1990s are still pulling income without benefit of a CMS. There has to be a compelling reason to make that kind of transition. If those reasons exist, then make the change, take any lumps, then SEO after the change.