Website SEO Optimization Experiments

I've often been curious about the black magic in SEO. I was reading a review of a new SEO tool that, among other things, directs you to tag any of your older content with relevant keywords if they didn't have them already. I happen to have a bunch of old content that I pulled in from various old blogs to centralize here. I've decided to do a little experiment to see if tagging older content on this blog has any effect at all on search engines. Took me a while to tag all the posts, but I'm guessing it will have little to no effect. We'll see.