If you opt to subscribe to the new service at $9.99 per month, you will still access all of the music you have already purchased. The benefit of the paid subscription would be that you could stream unlimited songs that you have not already purchased. It will be up to you to determine if it is worth it to you or not. Apple is offering a 3 month trial for free. I would suggest that you try it out just to see what you think.
Also, the link below leads to a chart that shows what you get free, vs what you get with the paid subscription.
https://www.apple.com/apple-music/#mn_p