Make your changes slowly over time. Making a drastic change is a very bad idea and will potentially kill your results. Doing one thing at a time and watching the results will help you in figuring out which effects are bad and which are good.
I have a quick question for the SEO guru's of the board.
We've had the same website for almost 4 years now. The content, page names, etc have been the same for all this time.
My main concern is with some pages being removed or renamed. Do I just let Google sort that out on it's own or should I be playing with redirects, etc?
i am not a guru but recently also changed the structure of my website and can say for sure that you should "play with redirects" not to lose traffic coming from already indexed pages. Google will certainly sort this out but it will take plenty of time and will effect your seo ranking