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" Launched in 2003, back then it wasn't such a bad idea to use keywords/tags in the URLs, so I did that." - it's not necessarily a bad idea now - I'd question why you feel the need to change them? Shorter is often better though. If you do go ahead, 301 would be the best practice. Canonicals pass link juice too but they're not always honoured. A couple of extra things - make sure to update all of your internal links to the new URLs as millions of 301s could negatively impact the speed of your server. And check a sample of the old URLs just to make sure everything is working as expected.

This may be a little late to the game, but, Dynamic Page titles in IMO are not great. However, Will including page Views in the title benefit the users? Maybe. But I don't think that one or two dup page titles will kill your SEO. Worse case, could you mod your system to not allow Title which have already been used?