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Why not update it moving forward? So, rather than changing the old threads that are historically 10 posts per page, just switch to 20 for new threads and leave the current ones 'stable' as they are...That's more like I was thinking if it's possible for you to do with your current set up. More text and better content per page might raise the value of the current "page 1" pages but if you saw a drop in ranking due to 404s then you know what to expect as far as damage AND damage recovery.
Specify a View All page. Searchers commonly prefer to view a whole article or category on a single page. Therefore, if we think this is what the searcher is looking for, we try to show the View All page in search results. You can also add a rel="canonical" link to the component pages to tell Google that the View All version is the version you want to appear in search results.
problem with a view all with forums is some forums have lots of 1000+ post threadsYes, I would be very cautious with even having View All option available to users, let alone Googlebot and thousands of other crawling creatures that will hit it. I don't know about 1000s - I tend to split those up - but 200 or 300 responses in a thread slow it down significantly and 500 in one thread could bring the server to its knees - had this happened before. And what's sort of ironic - as soon as a troublesome page like that appears on the site, Googlebot starts to hammer on this page and make the load / speed situation site-wide or even server-wide worse.