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If I remove everything and replace it, how will google view this big change? There are really just too many crappy content pages to redirect. Will I just have to hope and pray that they will eventually remove the old stuff from the index and include the new stuff, or is there something I should be doing to make this as easy as possible?
If it is ranking well, my suggestion would be to keep the pages that were ranking well for their keywords, but have the links to them on your site less visible to the visitors.
It's my opinion that well-ranking pages on a site help "pull" the other pages up in rank. I can't verify that, but it's been my observation. Therefore, keeping the well-ranking pages will help the others, and eventually you can drop the old pages completely.
Of course this is only feasible if the new design is compatible with the old so the old pages don't look that much different from the new.
The site content and quality is much better, Google has picked up the changes, and the SERPs changes have been modest and mostly favorable.
I admit, had this been a higher traffic site, or a huge actual or potential money maker, I would have been more cautious. And it is possible that for more commercially competitive keywords the results would have been different. But at least in this anecdotal example, huge changes were implemented with the user in mind, and Google seemed unbothered.
The site has huge potential, but as it is now, and has been in the past, it is nothing, so I'm not concerned about losing what I have, just don't want to spoil things for the future.