Well, considering that our traffic hasn't recovered, the short advice I would give is "don't change URLs." But our pages were performing really well before, although we didn't understand that until later.
What I would do differently:
I would research and document the current pages' value before making a move:
- how many click/month from google/yahoo do they generate per month?
- how many unique visitors is this worth per month?
- what keywords are the pages ranking for (especially: are these pages ranked #1 for any terms?)
- how well-linked are these pages from 3rd-party sites?
- do search-engine users click through from these pages to other areas of the site?
I would make sure the company management understands the risk of a 6-12 month dropoff in pageviews.
I would secure a budget for staff to pursue linkbuilding strategies following any URL change.
I will in the future be very cautious about comprehensive URL changes. For our next move, we're going to start with the pages that have no pagerank, no external links, and which drive little to no traffic. They have no traffic to lose, so our opportunity for gain is much greater.