Today I found out about and was able to cancel a purchased ticket.
Tickets purchased can be refunded online within 2 hours of purchase.
Scroll down to bottom left side of Renfe website and its possible. You will need both record locator and ticket numbers to fill in form.
I think its on all tickets within 2 hours of purchase. The ticket I bought was a Dorato ticket.
I was told this by a Renfe rep.
You are right that the guarentee is a good on arrival times. A train I was on to Barcelona was late and I got the whole purchase price refunded easily and quickly all online.
And I did not pay a change or cancellation fee. I cancelled the ticket completely. Then I started all over and purchased a new one. I also was able to pay with a US American Express.
Thanks for posting this information Ann!
According to the Renfe website, one can cancel or change a train ticket (except discounted tickets that are generally not refundable or exchangeable) up to 15 minutes prior to the train's departure time, but there will be a 15% cancellation fee deducted from the purchased price.
And as Ann has mentioned,
"Tickets purchased can be refunded online within 2 hours of purchase".
Tickets can also be canceled in person at the train station.
You can read about the cancellation conditions on Renfe-FAQ , however it's only available in Spanish.
Enjoy your time in Spain!