Ask HN: How to convert 5 star ratings to up/down voting?

I've got a site with close to a million ratings on thousands pieces of content. I'm redoing the site and I want to scrap the 5-star rating system and go for a like/dislike thing--sort of like what YouTube did.

The system will display how many votes there have been and the score (up votes minus down votes).

I want to keep all those ratings from the current setup, and still be at least a bit accurate with the scores (aka content that had higher star rating to have an appropriately higher score--of course this also depends on the number of votes which is why I don't exactly know how to go about it)

Suggestions?