You will not see a difference in video and there will be no better sound. The only reason to switch to HDMI in your case would be if you need to free up a digital audio connection for your surround sound receiver. You can use either digital coax or optical cables for your audio if you have either connection available. The sound quality with either of these will be the same.

I know there are technical differences and some will point out the differences. In reality, you will not actually be able to see or hear it.