incrediBILL - 5:49 am on Aug 14, 2013 (gmt 0)
Assuming the pages had mostly the same keywords and basically the same content, I would've thought the pages would've stayed but if you're talking complete rewrites, it's just like you posted a new page IMO.
When you have updated content it's often best to post a new page and then simply insert in the old page "UPDATED: MM-DD-YYYY <Link to update>" and leave the old page completely as-is to maintain it's position.
Not knowing what you posted and how it was previously linked with anchor text, perhaps Google simply saw an apples and oranges issue that the things didn't match well enough and it tossed it down the well. Sites and content pages change hands all the time, fall to parking services, etc. and do not maintain their status when these events happen. Perhaps your changes triggered that part of the algo that thought the pages were no longer part of the same set.