In other threads here on WW some of the senior folks have said minimizing the restructuring and keeping the number of 301's down has a value. This was in reference to traffic recovery post change roll-out. A few have suggested to not do it all at once as well based on their seeing a slower than expected recovery. Others suggested lining up links to new page location ahead of time before switching over. It goes without saying to update sitemaps frequently as you work as well.
How big a site are you talking: 10's of pages, 100's or 10's of thousands?
the size of the site is between 50-100 pages so not massive. the questions is what would be the effect of having 2 designs in place so the main design would be changed over but the inner pages changed over time, perhaps 1-2 months (I'm talking effects on the SE's not the punters which I know could be a little confused, but needs must)
page names/ links will be the same, its just the site needs an update and due to work loads this can't be all done in one hit