I would personally recommend you rewrite the content. You can never be too sure with google. For once, even if is good content, who knows if google keeps track of all previously published content (meaning if you are to repost the exact same content it can be seen as spam).

If I was you I get either, brand new content or do a high quality rewrite of all your content

Agree with Greg and err on the side of caution. Many will just come on and say wait until it is no longer indexed by Google and then use it again - but I think they are smarter than that. You also run the risk of other's having already snatched your content and they'll end up beating you in republishing in the SERPS for it.

I know it seems paranoid but I'm pretty sure google has tons of content within it's own archives (including all that within the supplementary serps) that it's not going to forget about. Maybe that's for the future and storage capacity issues hinder them exploiting it - but surely they are wising up to this?

Having said all that - I've used a ton of old content that I know was indexed back over a few years and I've had no problem with it - I suspect with the increasing use of repurposed domains and building of networks using dropped and expired domains this is something that Google will target shortly. Just a hunch but for now you might just get away with it.