Rewriting Content on Bought Site

I'm buying a site that's well established and earns about $500 a month in revenue. The design I know I can improve on without sacrificing rankings. However, what about the content? While readable, the content on the site isn't the best. Would it hurt it's current rankings if I rewrote the content? Also, would 301 redirecting a URL to a new one (to improve content organization) hurt rankings as well? For example, redirecting /page to /category/page Any advice would be heavily appreciated!

In my view if the new content's keywords density is kept the same and its uniqueness is improved, it might not disturb the rankings.