Can I take an earlier HSR train if I did an advance booking?

If I did an advance booking for a HSR train and got the 35% off early discount, can I take an earlier train on the same day, or must I take the exact timing I booked for?

You can exchange the ticket at the ticket counter, but they might ask you to pay the difference in fare. The first exchange for HSR tickets is free (no service charge), any other changes, they will charge you a fee.