Just thought I'd add that if you have to redirect pages, make sure you do a 301 redirect & not a meta refresh...I'd recommend doing it through .htaccess.
Here's a link if you don't know what I mean.
Also, if the content remains similar (same niche) and the backlinks remain in place, you shouldn't have a problem with losing placement, imo. If anything, you may experience a temporary drop as the site is revised & reindexed by the bots...but nothing permanent, even if you do change it up quite a bit.
I've never bought domains with established PR, but I have completely reconstructed pages without loss of placement in the SERPs. All in all, I'd only do it if I were targeting (quite) similar terms or phrases.
Good luck.